Took me awhile to actually get to this game, but yeah. I finally beat this without pressure of anybody around me after I left it alone for awhile...
Mass Effect 2 is a direct-sequel to Mass Effect. As a direct-sequel, you are able to upload your game data from the previous game and use it for the newer game. For some cases, doing things like this in other games only lets you transfer gold, status, and other materials over. But for Mass Effect 2, you're able to change the entire out come of the game (in some cases.)
IF you didn't transfer over your game data, ME 2, has the same, systems like in the original ME where you choose your type of character, social status, class, and other character customizations. The default story, whether you like it or not, is that you sacrificed Kaiden during the last game. The krogan, Wrex, is also dead by default. That means Ashley is the survivor in here, as well as everyone else I didn't mention. You have a new team, besides Joker (who's irreplaceable,) as well as a new ship, new enemies, new everything.
Gameplay-wise it's not really much different. That can be good or bad. There's some improvements to minor things, like ore mining and armor customization. But battle-wise, nothing has changed.
When you beat ME 2 there will be an option for you if you want to continue playing or start over again with most things intact. I highly recommend you start over. Why? Everyone keeps their levels, guns, and Shepard gets to learn a new skill that can be outside of his class. I'm a soldier with a biotic skill. Not bad...
Overall, the game is still running good. There are strong hints of a 3rd game in the works inside the 2nd game, especially during the loading screens. And if you take my advice on the ending game part, you be off to a better start in the 3rd game later. I give this game 4.8 out of 5.