ME2 is a good action/adventure game, but don't mistake it for an RPG.
As an action/adventure game, ME2 is a good, solid buy. Decent length, good combat, interesting setting, likable characters, and great voice actors. But to call it an RPG is a stretch of the truth; you only gain experience after completing missions, and the experience is used for upgrading class skills, not boosting your stats. The only problem I really have with the actual game mechanics is that the morality system is as shallow as the toddler pool at a swim club: you can make "good" or "evil" choices which make your character more "good" or "evil", allowing you to choose more effective conversation options, but that's as deep as it goes; other than the different dialogue, the conversation will turn out exactly the same way. If you played a "good" character, there's no need to play again to see what an "evil" character is like; the gameplay is the same in all respects.
On the other hand, ME2 makes a lot of basic mistakes from the RPG point of view. If the description of the experience system above sounds bad, get this: there's no other way to get experience other than by completing missions, and not only is the level cap too low for you to be able to fully upgrade all your class skills, but your AI partners only get points to spend on upgrades every OTHER level!
That's not all; completing missions is the only way to get money, and there are a finite number of missions in the game. To say you have to watch what you spend is an understatement: If you buy ALL the DLC, get ALL of the completion bonus money from an imported Mass Effect 1 file in addition to the bonus money for using that file to beat ME2 as well, AND do every single mission in the game perfectly, you can just barely afford all the essential upgrades (that doesn't include decorative items). That's right, completionists: you're playing an "RPG" with a finite number of monetary resources. Getting back on the subject of DLC, I get the sense that a lot of the game's content was chopped to be sold separately, as a lot of the content has to be bought and the game seems pretty bare-bones without it.
Finally, ME2 has a trait which many gamers find unforgivable in RPGs: event triggers which force you to advance the story before you're ready, making certain things lost forever in the process. And notice, I said triggers, not trigger. There are several of them. Had ME2 been designed as a standard RPG rather than an action/adventure game, these flaws would have made it deserving of a 4.0 at best. But again, the game is not actually an RPG, and that's what ended up saving it. On the other hand, as a squad shooter, it doesn't have as much replay value, especially in light of the previous issues. Viewing it as a squad shooter, it's fun the first time, but it loses its magic shortly after.
What really kills this game for me is that it's clearly meant to be played as part of a series. All the replay value comes from playing the game again making different choices in each entry in the series. While that is a nifty gimmick for the series as a whole, it means that individually, the Mass Effect games suffer, and that problem is only compounded by the aforementioned DLC issue. I'm sure this game might have seemed better if I owned the rest of the series, but I'm not about to buy two other games I don't particularly want just so I can enjoy this one to the fullest.