On the whole, a great sequel that improves upon many of the original's shortcomings
As with the first game, it's the plot and characters that drive the whole experience. You're back with some really interesting new team mates, as well as some old favourites like Garrus and Tali. The wierd thing is that many of the missions are personal help quests for each of your team mates to earn their loyalty. This is an interesting idea, but in reality falls a bit flat because it really makes no difference to the final mission if your team mates are loyal or not. However, it does provide insight and fleshy backstories for your comrades, so it's still enjoyable to play them.
As with the original, if you keep your eyes on the prize and ignore all optional missions and side quests, you can beat the game in less than 15 hours. However, if you want to do every mission and explore every square inch of the galaxy, you could easily double that. The difference with ME2 though is that you will want to play the side quests this time round as they are for the most part interesting and offer up worthwhile rewards, unlike ME1.
The combat has also been greatly streamlined and tuned up. We now have a cover system that works all the time, and team mate AI is nowhere near as dumb as the previous game - they will occasionally pop up between you and your target and then complain that you're shooting them, but mostly they stay out of your way. Gone is the clunky equipment system, you now have one of each weapon type which you can upgrade as you progress. It's all good.
There are still a few let downs though. I noticed several bugs where characters would twitch about during cutscenes and conversations, or enemies which floated a foot above the ground. These glitches didn't occur often, but considering the polish on the game's visuals and sound, when they did pop up it was a little jarring.
Gone are the regular and lengthy elevator rides masking load times between levels. You'd think that would be a good thing, but they have simply replaced these sections with loading screens which are even more boring than the elevators. What's worse is that you have to endure these lengthy load times even going between decks on the Normandy - for some reason they have done away with the stairs!
These are minor annoyances really, and one thing that more than makes up for them is the ditching of the horrendous Mako vehicle sections that bogged down the first game. In ME2 you simply get dropped in a shuttle straight down to your destination. It's wonderful. The framerate problems and freezing during autosaves from the first game have also been dealt with - ME2 runs smoothly throughout, and is visually even richer and more gorgeous than it's predecessor.
So all in all, anyone who enjoyed ME1 should love this, and anyone who didn't enjoy it because of it's numerous bugs and shoddy gameplay should also give ME2 a go. I doubt you'll be dissappointed.