I finished my first play-through of Mass Effect 2 and, overall, I enjoyed it. To explain why I liked it, it's probably best to compare it to the first game.
The first Mass Effect had its moments, but it was a deeply flawed RPG. It's five biggest problems, in no particular order, were:
1. Tedious resource-gathering missions (minerals, Turian insignias, etc.) that added nothing to the game.
2. Side quests that all took place in the same damn mine or prefab house.
3. The Mako, a.k.a., the worst video game vehicle ever.
4. The cumbersome item system, which gave you both too much (lots of variations on the same gear with only minor differences) and too little (only allowed to carry 150 items).
5. The computer-controlled allies often did stupid things if you did't micro-manage their tactics.
How well did Mass Effect 2 address these problems? Well...
1. The resource-gathering (done by scanning a planet rather than driving around) is actually essential because the resources are used for upgrades. On the other hand, scanning planets is tedious as hell. At the very least, you don't have to scan every planet since there's more than enough resources in the galaxy, but it still slows the gameplay to a crawl.
2. Side quest locations are much, much, much more varied in both their design and mission objectives. There are fewer quests overall, but they offer far more rewarding gameplay.
3. The Mako has been replaced by the Kodiak, a shuttle that simply drops you right next to the target location. No trying to drive over mountains.
4. The item/weapons menu is effectively gone. Characters carry whatever weapons they have a talent for, and there are only 2-3 versions of each type of weapon. Much less fussing over what to turn into omni-gel.
5. The computer-controlled allies are still pretty damn stupid, but it rarely hurts the combat experience.
So it's a big improvement, especially with regards to side quests and item management. The resource-gathering, however, needs to be either completely revamped or chucked out entirely. I want to shoot aliens and save the galaxy, not mine for iridium.
Despite my complaints, I would recommend Mass Effect 2 for any RPG/sci-fi fan.