An amazing RPG that gives more then what you pay for it. Buy it.
The story is also engaging, but the real value lies in the side missions that your crew give you. These can be pursued at leisure, and give a great deal of insight into these characters that is rarely given in RPGs. These experiences make the characters feel like people that you know and help to immerse you in the game. Getting to know the characters also makes it very emotional should they die later in the game, further immersing you in the game.
As mentioned above squad members and the main character can die permanently in the game. No this does not mean that if they get shot by some random goon that you cannot use them again, if that happens they can be revived. Rather this means that in the final mission if certain decisions are made then the deaths of characters will be triggered. This will cause them to die permanently. The game will continue, however this character will be unavailable. This gives much more weight to the choices you make in game, as you are actually taking the lives of your team into your hands. Should you screw up too badly it is possibble that everybody in your squad and the main character will all die, rendering the save file unloadable for mass effect 3.
Another one of the features that makes mass effect 2 so addicting is that you can load your files from mass effect 1 into mass effect 2. This gives an extra sense of depth and meaning to the storyline. In most RPGs the choices you make will have minimal effect on the game world, or a gamechanging effect that can only have one effect giving it a hollow feel (ex. people are always grateful if you save the world). Due to the import feature the decisions that you sometimes made without thinking in the first game have a massive effect on the second game. These changes are made even better because they can have two or more different outcomes making the choices feel like they have meaning.
(spoilers - do not read if you have not beaten mass effect 1)
For example there is a moment in mass effect where you could kill Wrex as he considers changing sides. Doing so makes several missions more difficult, and the atmosphere of the Krogan homeworld far more hostile.
(end spoilers)
Even some minor decisions that are often made without thinking have an effect on mass effect 2.
(minor spoilers)
For instance when dealing with Conrad Verner (annoying fan dude) in mass effect 1 I chose to threaten him to get him to stop annoying me, but then I meet him in the second game and he has turned into this vigilante dude who doesn't help and has taken a very arrogant attitude. He ends up causing trouble at a bar on one planet. It made me sad that I had made this funny character so cold and heartless without even thinking.
(end spoilers)
I then wanted to go back and change my actions, but you can't as it happened in an entirely different game. This gives Mass effect a sense of meaning that few RPG's have.
In conclusion you should buy mass effect 2 as it is awsome.