Excellent game! Blows Mass Effect out of the water. I will try to avoid spoilers as much as possible.
Bioware did an excellent job with Mass Effect, but with Mass Effect 2, they really outdid themselves. They pulled no punches with this epic game.
I will try to avoid spoilers in this review, but please understand that to give a good review, some spoilers may exist.
Two years after the Battle of the Citadel, Commander Shepard is thrown out of the pan, and into the fire, as he is given a new ship, a new crew, a new life, and a new mission. The Collectors have been abducting human colonies, and Shepard has been sent to investigate, and stop them by any means necessary.
Travel from planet to planet, righting wrongs, or wronging rights, as you explore the galaxy, discover the secret behind the Collectors, and rally your forces. This time, your team does not just fall at your feet by chance. You must travel the galaxy and recruit your team yourself. Commander Shepard's natural talent as a leader is put to the test as s/he must complete missions to recruit the elite team s/he needs to take on the Collectors.
~Upsides~
The story becomes even more epic, as Commander Shepard gains new possibilities of action. You can now interrupt a conversation with paragon or renegade actions. These new interrupts include comforting the emotionally hurt, interrupting someone's talking by shooting enemies mid-conversation, shoving someone out a window, or even simply interrupting someone with your own responses. This new interrupt system allows you more control over Shepard's actions and options. This alone adds an epic new twist to the paragon and renegade personality system.
The paragon and renegade options become even more loose and relaxed, allowing more flexibility with Shepard's personality while still retaining the fluidity of which it executes. This allows the player a very strong control of the personality of their Shepard.
You may import your Shepard from the first Mass Effect game, thus allowing your actions and choices in the first game to effect the galaxy in the second game. However, you can still choose to start from Mass Effect 2, using a preset of default decisions.
The graphics get an upgrade, but the upgrade is done smoothly from the first game. While the upgrade is very good, it is not so much so that it would ruin the immersion if you are playing both games straight through.
You may now vault onto or over low cover or a low rise. Mass Effect offered this feature originally, but only with certain very specific objects that the developers actually intended you to climb on, thus limiting when you could do this. In Mass Effect 2, this feature is upgraded. You may now vault on, or over, an object at will, as long as it is low enough, and does not put you someplace you should not be. The latter case being a very rare issue.
Mass Effect was fairly realistic in Shepard having to earn the loyalty of his crew, but in Mass Effect 2, this becomes even more realistic, as Shepard has to actually do something terribly important for each of his squad members, in order to secure their loyalty.
Once again they balanced the difficulty settings very well. Casual lets you enjoy the story without having to be a very good gamer, while hardcore presents the challenge that hardcore gamers would like, and yet there is still the insanity setting for those very severe gamers who like the best challenge their game developers can offer. The wide range of difficulty settings allow the user to get the most out of the experience.
~Downsides~
The planet scanning and mining process was boring, tedious, and very annoying. They had a nice idea going, gathering resources and using them to build the upgrades, but I think they used an annoyingly inefficient system.
There is a glitch that can cause Shepard to end up on top of an object that you intended to take cover behind, or if you simply bump into the wrong spot just right, you can end up on top of the object and get stuck. I have been unable to determine the cause of this, where you can do it, or how you cause it to happen. However, from what I have seen when it does happen, it appears to me to be a glitch in the model of the object, like there is a slight slope in a random spot that Shepard can walk on, thus getting on top of it, but it is generally so narrow of a spot, that finding how you got on so you can get off of it is difficult.
They took away vehicle exploration. No more driving like crazy, no more flying tank, no more running over enemies, and no more knocking over armature's.
Weapon and armor are far more limited in Mass Effect 2. You do not get nearly as many options or possibilities. I personally found this to be a downside, so I'm listing it here. There was not as much to work for in that department. This is one thing I personally found to be better in the first game.