Bioware's magnus opus is also the best game available on the 360. Its maturity, depth, and polish are second to none.
Knights of the Old Republic had a huge advantage right out of the gate: a large, extremely loyal fanbase thanks to the Star Wars franchise. It also possessed a creative advantage, since Lucasarts and its licensed products have spent decades building a universe with interesting history, characters, and locales. Mass Effect was Bioware's most ambitious project ever, requiring the developers to create a universe of their own. They succeeded wonderfully in the first title, and in Mass Effect 2, that astonishing universe becomes more layered, complex, and real as the game progresses. You'll revisit very few locations from the first game, so you get to see entirely new star clusters and worlds, with environments ranging from sweltering tropical jungles to icy wastelands. You'll meet and recruit characters whose personalities expand across the entire spectrum, from the icy, ultra-rational Mordin (a salarian scientist) to Grunt, the ferocious and bloodthirsty Krogan warrior.
This series is a triumph as an exercise in galaxy-building. The thought that went into it is obvious from the gameplay alone, but it truly becomes astounding when you look into the Codex and read the detailed articles on anything and everything about which you could have questions. The Codex even includes articles explaining the finer points of interstellar battle tactics. The history of the galaxy also includes several fascinating and tragic tales, such as the quarians' invention of the geth and their subsequent defeat at the hands of their creations, or the salarian-made genophage that ended the threat of Krogan military dominance by crippling the entire race's reproductive capacities. The ethical questions these stories invite, and that the game forces you to confront at certain key junctures in the side quests, mark Mass Effect as a truly adult franchise (much more so than the juvenile, even puerile, romance options available).
This is especially true if you played the first game and saved your save files. In that case, you can import your character, preserving the crucial choices made in Mass Effect, such as character deaths and the fate of the Council on the Citadel. These and dozens of other small choices, such as side quests performed for good or for ill, will influence the Mass Effect 2 experience. Sometimes the impact can be as small as an email sent to you sending you thanks for a previous good deed; sometimes it can lead to an interesting conversation with a character from the first game, or a unique side quest. Mass Effect 2 does a great job making you feel as if Mass Effect matters-that your actions in that game matter.
Set in this background, Mass Effect 2 weaves a strong-though not spectacularly original-narrative. The Collectors, an advanced and mysterious alien race that appears through the unnavigable Omega-4 relay, begins enslaving entire human colonies. Commander Shepard is sent to stop them, though he or she must first gather together a team of characters who combine strong combat, biotic (essentially the same as Force powers in the KOTOR games), and tech powers. The overarching structure of the game will be extremely familiar both to those who played the last game and those who have played any Bioware RPG; if you've tried the genre before and hated it, don't expect Mass Effect 2 to completely alter your perspective.
There are changes to the formula from the first game, and mostly for the better. Interestingly enough, almost all the failures from the first game were Bioware's tentative steps away from their traditional turn-based formula, such as real-time combat that resembled third-person shooters and an expanded side quest system. Rather than simply finding side quests on the same planets where your main mission was located, you could explore entire systems or star clusters that had nothing to do with the central plotline and land on strange worlds. Unfortunately, once you reached the surface, you drove around in a painfully slow, awkwardly controlled vehicle called the Mako until you found an enemy base, which miraculously had the same layout on every world you visited.
Rather than retreating, however, Bioware has forged ahead and fulfilled the promise of its original vision. Combat, which felt clunky and awkward in the first game, works incredibly smoothly this time around. The basic gameplay feels like Uncharted or Gears of War's pop-and-stop style of shooting, but you combine it with squad-based mechanics akin to Ghost Recon: Advanced Warfighter and a set of special abilities equivalent to magic or Force powers in other role-playing games. Your guns even require reloading. The wheel controls from the first game return, and you'll still make occasional use of them, but the improved assignment of hotkeys means that you'll be able to fight entirely in real-time. These improvements cannot be overstated, vastly increasing the kinetic pace and intensity of the fighting. The level design is also improved from the previous game, though it doesn't rise to the complexity of the best stages from this generation's premiere third-person shooters.
As for the side quest system: this too has been revamped entirely. There are still plenty of non-essential planets and star clusters that you can explore, but you scan the planet first before landing. If there is an "anomaly," then you will be able to find it and land for a side quest on a unique map (no more endless wastelands). If there is no anomaly, you can scan and then mine the planet to cultivate resources, which you'll use to upgrade your weapons, armor, and abilities. You can even upgrade your ship, which will become essential in the endgame. Unfortunately, mining is also the biggest bore I've suffered in awhile. As a principle, minigames should be short and have at least a little hook to get you excited (such as a time limit); mining is the exact opposite, as it takes a long time to do it right and get the maximum amount of resources out of a planet.
This flaw is all the more startling because it's pretty clear that streamlining was the mantra for this second go-around. Not only has the side quest system and the combat been streamlined-the character customization and progression options, along with the item system, have been vastly simplified. This is going to be a point of contention with some gamers. Unlike in Knights of the Old Republic, where you had a set of six attributes to improve, along with skill points to distribute within a list of dozens of abilities and powers, your character will basically have four categories that they can level up. Three will be abilities, such as biotic powers, tech powers, or ammunition buffs. And one will be your character's class evolution, which increases attributes across the board, such as health, shields, and weapons damage. There are no item drops from dead enemies, either, and the variety of items is miniscule as a result. Every once in awhile you will find a new weapon or new piece or armor, but each category of weapon only has two or three varieties. For people who loved finding unique items, which surely encompasses a large number of role-playing fans, Mass Effect 2 may be a little disappointing.
However, I found this to be quite consistent with the mythology of the series; unlike in Star Wars, where the mystic aspect is much more important and one can conceive of special Jedi robes or armor pieces imbued with the power of ancient Force users, Mass Effect does not have room for "special" items in any consistent sense. There are a few modified weapons that only certain party members can use-an overpowered shotgun only for Krogans, for instance.
The streamlined design is matched by a more polished presentation this time around. While the original game could be gorgeous, it also suffered from noticeable framerate drops, horrendous texture pop-in after reloads, and a host of other stability issues. This time around, the stellar voice acting, camerawork, and cinematics from the first game return, but they are matched by seamless in-game performance. Some reviews have claimed that they still got occasional glitches and problems, but none of these manifested during my roughly 25-hour long playthrough.
If anything negative can be said about the game, it is something that may be endemic to this generation. As budgets increase exponentially, and every single small piece of game-world becomes much more difficult to design due to the increased level of detail and interactivity we demand, there is a certain narrowing to the genre. This isn't felt strongly in shooters, which have always been linear, but it impoverishes a role-playing game noticeably. Already, we've seen the elimination of towns from Final Fantasy XIII, simply because the resources to build such non-linear environments were unavailable. Mass Effect 2 remains open-world in the tradition of Knights of the Old Republic, but every now and then you feel that squeeze. The side quests are much more abrupt than earlier generation Bioware efforts; some of them are as simple as talking to a person to receive a mission, and then talking to a second person 10 feet away to solve the problem.
Mass Effect 2 certainly isn't a perfect game, but it's unquestionably the most advanced western RPG that has ever been built. Even compared to Bioware's Dragon Age: Origins, released a scant two months earlier, Mass Effect 2 seems light years ahead (and not merely because the setting is space opera rather than high fantasy). The fact that your choices carry over from the first game, and that your actions have subtle and continuous influences on the galaxy around you, make this an exemplar of what role-playing should be: the story that you experience is not the game's story alone, but your story.
UPDATE:
After a second playthrough, I find myself a little abashed about a few comments from before. When I played on a lower difficulty setting, it was quite easy. But on Insanity, this game is a serious tactical challenge. My only problem is that occasionally, the only way to win battles seems to be to manipulate environments and enemy AI for cheap wins -- hiding behind a particular set of boxes where heavy mechs can't reach you, then spamming tons of Warp spells until they die after a few minutes of agonizingly slow reduction of shields and armor.
My second playthrough is also as an evil character, and it's amazing how differently things play out as a real bastard. This is the sort of game you need to play twice.