Huge upgrade from the first game. More classes to choose from and your decisions actually matter.
The game takes place right after the first, but a cool feature added is for you to import your Mass Effect 1 save file to incorporate the 1st games ending with the 2nd game. A think this feature is amazing because its like your playing one very long game and all the decisions you made in the 1st game effect the 2nd one.
One thing I found that I'm not really sold on is that depending on the class you choose you won't be able to use every single gun in the game and your uses with biotics and technology are reduced. But again its spread out over more classes.
Buying this game brand new is a must because you'll get access to two free downloads, that you would normally have to pay for. One being a new added character and the other being a mission involving a search on another planet.
Bottom line. if you liked mass effect 1 your going to love this game. This a for sure GOTY candidate.