An outstanding game, though some issues rob it of the state of perfection.
The original Oblivion game is the fourth installment in the long running series of Elder Scrolls games, which focus on an ambitious open-world RPG. Oblivion kept what was best on the Elder Scrolls Morrowind (like open world, freedom of class creation and so on) while removing some annoyances (such as the fact that Morrowind relied on a percentage of chance to hit, you could spend a minute trading swings with an enemy to no effect.)
Elder Scrolls have an extremely solid character creation and development system, which basically allows you to play any role you wish, from an all out melee fighter, to a spell caster or a stealthy thief. Oblivion doesn't have like most RPGs a leveling system based on experience points, but instead relies on skills advancements. At character creation, you'll have to determine your major skills and your minor skills, major skills staring at level 30, while minors start at 5, you have a total of 7 major skills. To gain a level you must increase your major skills 10 times (whether it's the same skill 10 times or all of them a lesser amount is up to you), once that level is gained, you'll get the choice to increase your stats in different ways, for instance if you increased blades and stealth most you'll get higher bonuses if you choose to increase strength and agility. This of course makes character creation the single most important choice of the game, and for newcomers a very hard one.
Once the first part of the game which serves as a tutorial of sorts is over, you'll find yourself in the open with a main quest you're in no way forced to follow, you can go in any direction you wish, such as exploring the ruins just in front of you or going to other way into the capital city, you'll feel in Oblivion a considerable amount of freedom.
During the course of the game you'll be able to join different factions (Fighters' Guild, Mages' Guild, Thieves' Guild, Dark Brotherhood among others), and in those factions you'll be able to increase in rank through the completion of quests which will be given to you, these can be extremely fun or extremely boring in certain cases (very few thankfully). You'll also come across a spectacular amount of side quests, aside from those of the factions I just mentioned (at the moment i must be playing my 10th character or so and I still find new quests), for instance you'll come across a soldier of the imperial legion who'll ask you to clear a hide-out of skooma (some sort of drug) dealers, or you'll hear rumors of a ghost haunting the shores of a nearby lake and so on. Naturally completing these quests gives rewards (like permanent skill bonus for helping a gladiator find a proof of his noble blood).
The combat itself is satisfying, it basically plays out like a mix of an FPS and an action game, and it's interest lies with the endless possibilities for different classes and thus gameplays.
The main storyline is quite long and the first time through it will take you more than a few hours to finish it. The plot is interesting if somewhat cliché in certain cases and might no feel as fulfilling as Morrowind's storyline but it's still interesting to play it at least once. Parts of it are boring though, as at a certain point it'll degenerate into you going through weird portals into a world that basically looks like the typical idea of Hell, and where you must retrieve a stone at the top of a tower, it can be fun the first times, but after the third or fourth, you'll just want to get it over with which is a shame.
If you don't like quests and just want to explore the huge world offered to you, thats possible as well, you can explore Ayleid ruins, ruined fortresses and so on, each filled with monsters and, of course, loot. The downside is that the architecture being roughly the same, you'll get the feeling you're doing a dungeon you've already seen before long.
Sadly in this game all the good aspects have a little flaw that prevents it's full enjoyment. I mentioned earlier the character creation and development system which basically allows you to play whatever you like, it however suffers a MASSIVE problem. To get to this problem, it's better to mention first that the creatures you face increase in strength as you level, thus at level 1 a region might be populated by starving wolves, and at level 10 by trolls. The issue here is that the monsters level too quickly and if you're not careful, you'll soon be unable to kill your enemies. The way to avoid this is by putting none of the skills you'll use most as your major skills (for instance, my current character uses mostly Blades, Marksman, Destruction , Illusion and Sneak, and none of those 5 skills is a major one. This way my character only reached level 2 after nearly 10 hours of gameplay), this of course is ridiculous and newcomers won't know this and will find themselves playing underpowered characters almost every time.
Another problem that this game suffers is that once you've finished the main quest, become the master of all available factions, amassed a wealth that'd make the emperor jealous and gathered the best available gear, you'll still feel you haven't actually DONE anything. You'll look back and the world is still the same it was before you set out, no town as appeared, disappeared or grown, your actions don't seem to have actually been all that important, or have had any influence whatsoever. That lack of sense of achievement is disturbing and might take away the wish to play it again (it didn't for me but it's still sad in a way).
And it also suffers from the same issue Morrowind did, meaning the huge amount of bugs. The elder scrolls series has always been plagued by bugs, crashes to desktop and so on, and Oblivion is sadly no exception to this, I experience crashes to desktop roughly once or twice a day, and if that's small compared to other games, it's still annoying. Some quests are also broken but that's rare.
And now on to the add-ons, these are simply new quests, and in the case of the the Shivering isles a new continent. The gameplay itself is left totally unchanged which is a shame, as the developers could have used these as an excuse to modify some of the aspects of the original game that were flawed.
I'll speak of Knights of the Nine first, this add-on is extremely limited in size, and basically all it does is add a new quest. This quest however is quite long and more importantly is the best I've seen so far in the Elder Scrolls, the plot it interesting, some of the script used for parts of it are amazing (at one point you'll speak to a ghost miles above the world of Cyrodiil) at the end of the quest you'll get a set of armor that will increase in power as you level (all you must do is return to a certain place, put the armor on a stand and take it back) as well as a sword and a mace. At the end of the quest, you'll find yourself at the head of an order of knights but as with the original Oblivion you won't get that feeling of achievement other games give.
And now on to the Shivering Isles. This add-on has a much more considerable size than Knights of the Nine, and will take you to another world, the one ruled by the Daedric god of madness Sheogorath. Regardless of what others say, Sheogorath is my favorite NPC of any RPG yet (ten seconds into talking to him he threatened me with ripping my guts out and strangling me with them), he'll often roar totally insane sentences like « Cheese for everyone! » when you finish a quest. The shivering Isles are divided into two sections, Mania and Dementia which reflect basically all that's good (happiness and so on) and all thats darker (paranoia, mistrust) respectively. This reflects on the behavior of the NPCs you come across and the quests they give you (the Duchess of Dementia will name you Grand Inquisitor and will send you on an investigation to locate potential conspirators.)
What's very interesting in this game is also the change in look of the enemies you face, you won't find goblins, rats or wolves here, but creatures so strange they sometimes can't even be described and provide a welcome change in the monotony of the standard fantasy RPG monsters. The main storyline of the Shivering Isles is cleverly done and most of the quests are interesting setting aside the occasional « Go there and retrieve item X from place Y » that are present in every single RPG in history. My personal favorite is a quest where you reactivate an old ruin which is basically a trap for adventurers and you can watch then turn insane.
The volume of things to do is however considerably smaller than the one in Oblivion, and the size of the new continent is much smaller than Cyrodiil.
All in all, if you're into RPG, and for some reason haven't played Oblivion yet, then look no further than Oblivion: GotY Edition. If you've already played Oblivion, it's a tougher choice, it's certain you'll like this game but whether you think putting money into it is another matter, it might be cheaper to just buy the Shivering Isles as it's the real addition. If you're not into RPG usually, it's possible though unlikely you'll like this one.