Mass Effect 2 seems more like a test drive for, or a bridge for Mass Effect 3. Decent gameplay, but shallow script.
This game lasted 40 hours for me, and i've done literally EVERYTHING there is to do. Then it gives me the chance to go back and play some more but there's nothing more to do, save the DLC that will be launching with extra chars and what not, which i find boring once you've finished the game.
On the other hand ME1 took me almost 100 hours to finish and still there were a couple of things i haven't completed.
The main thing with ME2 is, it seems like a rerun with slightly different characters which are not that interesting, there is a pattern all the way through, one of the companions has a problem, you fly to the location, solve the problem, boom he/she now is loyal move on.
The old character are totally left alone and simply exist in the game (other than the problem part, which at least with Tali is actually a very good idea/scenario). Another good idea is the addition as your last character of an unexpected race (am not going to spoil it for you, if you don't already heard what it is). Another good idea, but shallow in the end.
ME2 is a much better played out game as gameplay is concerned, but scenario, pace, and immersion are almost non existent. Add to this an anticlimactic end, it's almost like the poor 2nd part of a trilogy with the minds of the creators set, mostly on its beginning and its end.
The underlying linearity of this game most probably will take away its replayability value. All in all it's a good game, just dont expect to be anxious and involved with your crew and the story like you were in ME1. For last i left something that disappointed me greatly. The absence of a substantial city to consider a capital and visit it over and over, as a base of operations. Citadel is not what it used to be, and it might actually be smaller than some areas that you'll find in distinct planets. Also the quests there can all be done in a single run/visit, with the exception of one or two fetch quests that if done right might actually let you finish all there is to do in Citadel in two gos. I was amazed, returning there randomly, to NOT find anything to do there, new, or unimportant or simply something.