Some minor spoilers probably. Might not want to read this if you've yet to play Mass Effect.
Mass Effect is a great game, no doubt about it. One of it's main selling points is the massive (excuse the pun) replayability the game has, as you can make different choices each time through. I took this to heart, and have played the game an unknowable number of times (My 360 harddrive erased itself a little ways back, so I can't check the clearsaves). I even got all the achievements, which everyone knows is a giant pain in the ass. And I have an important message for you all: Don't play Mass Effect multiple times.
It will ruin the original experience for you. When playing through the first time, the game seizes you with a sense of wonder and impact. It feels as if everything you does has an impact, and every choice you make seriously alters your play experience. This is a lie. Going back again and playing through really shows how little the game varies- the linearity is cleverly disguised, but it's still there. Oh? I can choose to save or destroy this colony? Well even if I save it I can't ever re-visit it and no one will ever mention what happened to it besides this Citadel elevator. Oh, I can kill or save this entire species? Bet THEY won't ever re-appear. Even though you're told that your choices will have huge impacts on the galaxy, they do not. Most of them don't even have a self-contained impact- someone will make a comment about your decision, then the story moves forward.
This even carries over to some of the dialouge trees. If you have a conversation and choose one option, then reload and choose anoher, the response is often exactly the same. There are a few honest-to-goodness moments that really do have an impact- the showdown on Virmire was probably the high point of the entire game for me- but these are few and far between really. They do certainly exist, but not to the degree you would think. And let's not even go into sidequests. Because really, I think we all know the problem there.
Bioware has said they want to tone down the amount of illusory choices in the second game, and make each have more impact, but faced with the prospect of a third game and having to carry over choices to that product, it's likely you won't be able to deviate too much from a preset path in ME2, much the same as in the first game, which is a shame, but probably a necessity to get ME3 out sometime before 2015.
It's ironic, but I think exactly because of the features that Bioware touts make Mass Effect more replayable, the game is better suited to be played once then set down forever. Becuase going back through again will tarnish your first playthrough, make the reality of the situation much more apparent than it was the first time through, when you were wide-eyed and full of wonder. In the case of Mass Effect, ignorance really is bliss.