A few weeks behind the times on my thoughts of ME2 but I don't exactly push myself to finish games these days, I take them at my own pace.
First off, make sure you've finished ME1 before you play ME2. It's cheap as chips on most digital distribution sites, and I've seen several $5 sales for it on Steam so there is no excuse for not picking it up. While you certainly can play ME2 without having played ME1 I wouldn't recommend it, as the story and writing is so good (in both 1 and 2) that you will appreciate the overall narrative better from having played them both.
Mass Effect 2 is good. It has some faults, but the positives much outweigh the negatives. And it's great to see a simultaneous PC/360 release on ME2 (other publishers take note please), whereas we had a delay of almost a year between the release of ME1 on the 360 and then the PC.
The storyline and writing, as with almost any BioWare game, is top-notch. BioWare continue to set the gold-standard when it comes to mature storytelling that doesn't involve catching them all. Yet it's not only the writing but the gray decisions that you have to make that make the game fun. Several times I was presented with a choice that I had to think over for several minutes before deciding what to do, whether this was something as simple as helping an old friend from ME1 or being asked to do some questionable things for my new employer. There are choices which are easy able to be pigeon-holed as good or bad, but there are always several other choices which have more murkier elements within them.
Classes are much better defined in ME2 than they were in ME1, and as part of that you actually have to think more about your squad composition than you needed to in the first game. In ME1 your class was significant but many of them blended together, to the point where it was difficult to tell the difference between a Vanguard and an Adept, for example. In the second game every class has one unique ability that only that class can do; Vanguards can Biotic charge towards enemies, propelling them through solid matter, Sentinels have power armour in addition to their regular armour and shields, and, when depleted, lets out an AoE energy blast which knocks down any nearby enemies; and Infiltrators can both cloak and slow down time slightly, giving them extra time in which to line up the perfect headshot. These are some of the best examples, and they do help to differentiate classes in a much better way than was done in the first game.
Better classes also have the knock-on effect of your party members being more important choices than before. In ME1 most people would just take whichever two squad members were their favourite to every mission, regardless of which enemies were involved or anything else. In ME2 you have to mix and match to a much greater extent. Know you're going to fight Geth? It makes sense to have at least one Tech specialist with Overload, or maybe even two. Hitting up some mercenaries? Maybe make sure you have a Soldier who can tear through human targets. It adds a little more depth to the game, and is something that was certainly lacking the first time around.
The combat is also much improved from ME1. ME1 wasn't a terribly bad shooter but then again it wasn't a terribly good shooter either. In this second iteration the combat has been improved significantly, and enemies now drop thermal clips which effectively serve as ammo instead of using the whole "thermal heat" concept from the first game. Combat feels punchier, powers feel more unique and the whole cover-based fighting system works well. One drawback is that the game does telegraph to you when combat is about to begin, because of all the chest-high boxes and ledges which conspicuously appear at these areas. This does break the immersion a little, but other than putting boxes everywhere I'm not sure how this could be avoided. It's just something you need to accept.
The clunky inventory system from the first game is also culled, in favour of an upgrade system that improves your current weapons over time as you collect resources from planets by launching probes from orbit. The mining mini-game is somewhat annoying, but it's mining, exactly how do you make that fun?
In short, I fully expect Mass Effect 2 ("Mass Effectier" was the rejected subtitle, I believe) to be in consideration for Game of the Year when December rolls around. It is an extraordinarily polished and well-written cover-based shooter in an amazingly fleshed-out universe. Seriously, the head writer over at BioWare is certainly justifying whatever monetary compensation and/or weekly selection of nubile young women they are throwing his way, because this is some of the best writing in a game so far (along with BioShock as another top example).
The ability to import an old save from Mass Effect 1 into Mass Effect 2 is a very nice touch as well. The game remembers "hundreds" of decisions you made in the first game and they come back to either bite you in the ass or give you a peck on the cheek in the second game. BioWare have confirmed that this will also be in the case in Mass Effect 3, where you can import an old ME2 save into the game, so be sure to hang on to your saved games between formats as there is nothing quite like being able to play the same character through three distinct games; it certainly adds a sense of longevity and gravitas to the proceedings.
I won't be as crass as to award ME2 a score, but simply accept that it is an excellent space opera that improves upon the shortcomings of the first game in almost every single way. If you haven't bought it yet then what exactly are you waiting for, Christmas?