There are two types of gamer in the world. The ones who can handle a game with an amazing, enthralling story set in the outskirts of our galaxy and those who only play GTA and ghost ride prostitutes off of bridges in burning cars (not that there's anything wrong with that). If you fall into the first category, the Mass Effect series is one of the most enthralling series to come out for PC and Xbox 360, well, ever. The only other games that come close are Dragon Age: Origins and the two KOTOR games. Interestingly enough, every one of these games was created by Bioware. Maybe I'm biased or maybe they just make the best story driven RPG games ever made.
Mass Effect 2 picks up where the first Mass Effect left off. Even if you haven't played the first game, however, you can jump right in. There is a huge attack that leaves your ship in pieces and your fate is unknown. If you've played the first one, you'll be in horror that your beloved ship is history but for newcomers, this starts you off on an amazing saga that you won't likely be able to put down for many hours.
The main plot of the game begins to unfold as you are dispatched to investigate why entire colonies of humans are simply up and disappearing. You could easily spend a couple hours wandering around, finding equipment and talking to people but eventually you get another ship and the galaxy is yours to explore.
What makes Mass Effect 2 and its predecessor very unique are the dialog choices. However you talk to people will have a dramatic impact on your game directly and indirectly. If you talk nicely to the people you are working for and those you come across in your travels, you'll have different responses later in the game from your crew members and others. They'll also remember what you've done in the first game if you have that save file (though not needed). This really gives you replayability and drastically alters some aspects of your game. Whether you kill certain people or let them go will also change things significantly later in the game.
The Characters are really what make this game shine. They are very believable and you may actually grow to care about (or hate) them. There are even some opportunities to betray them or even have sex with them after a long courtship.
Mass Effect 2 is a game not to be missed. If you can't afford it right now, pick up the first Mass Effect which should run you $20 or so for PC or Xbox 360.