Bioware outdid itself with this masterpiece. It's right up there with its classic Baldur's Gate series and KotOR.
Gameplay: The game is largely a third-person shooter, with a good dose of roleplaying and exploration. The shooter part is entertaining and accessible and gives each battle a bit of flare. Over the course of the game, you gain a number of high-powered weapons and upgrades, which make firefights more dramatic. The roleplaying elements mostly involve upgrading a few abilities of your character and his/her allies, as well as making various decisions about quest plots. The characters in the game are well-developed, and your teammates have their own stories and quests for you to accomplish. There are dozens of quests in the game. Some are part of the main quest; others are related to your teammates. Additionally, exploring the galaxy can open up a number of optional quests, which are typically short but imaginative. Many of the quests have multiple stages and help to flesh out an intriguing story or character. The game is worth playing a second time just to see how differently some of the quests turn out based upon your decisions. The ending, which is exciting but inconclusive, obviously sets up the sequel (ME3).
The team structure of the game works quite well. You can usually take 2 teammates on missions. For the most part, they hold their own in combat and don't have pathfinding issues. Most teammates are talkative and add an illusion of realism to the game. The diversity of species in your squad makes for some interesting matchups, though I found all of my allies likable. You can even start romances with selected characters, though the game seems to limit you to only one such coupling per playthrough.
Graphics: ME2 is a gorgeous game. Character models and animations are life-like, and the environments are beautifully rendered. The game has a lot of cutscenes which could be right out of a first-rate sci-fi movie. The game is well optimized, and I had no trouble playing it with a Geforce 8800GT video card on 1680x1050 resolution (mostly high settings). A lot of care was put into creating the lush visuals, which give the game a very immersive quality.
Sound: Like the graphics, the game has high-production values in the audio area as well. All of the voice acting is good, and the weapons, ships, explosions, and so forth are convincing. The soundtrack is fine, and the music generally fits the mood of events.
Technical issues: The game is extremely well polished, at least with the 1.02 patch applied. In over 35 hours of gameplay, I did not experience a single crash, freeze, or other technical error. This is rather amazing for a PC game. ME2 comes with a decent manual. Very importantly, the game has a very reasonable DRM system. You need only have the disk in the drive--no need for any online registration or authentication that would otherwise make this single-player game dependent upon an online server. I really do hope that they keep this system for ME3.
Replayability/Value: Given that I bought the game at Amazon for $15, it is one of the best deals out there. It is worth the original asking price of $50, so I definitely got my money's worth. The game's flaws are minor, such as a rather conventional main storyline, some gratuitous profanity, and a bland but necessary mini-game involving mining planetary resources. However, the cinematic and technical achievements of the game are such that these minor flaws are just quibbles.
I will replay this game again at some point (it takes about 35-40 hours to complete), but for now I'm looking forward to ME3. I highly recommend ME2 to anyone who likes shooters, roleplaying games, or simply a well-told and exciting story. You do not need to have played the first game in order to understand and enjoy this one.