I'm far enough into ME2 now to have formed somewhat of a better opinion - I definitely enjoy the game. There are some plusses, one BIG minus and some changes I'm not sure about compared to ME1:
Plusses:
+ Better implementation of the paragon/renegade thingy - liking the optional conversation interrupts.
+ More dynamic combat - I think the companion AI is better than in ME1, and actually surpricingly good.
+ Improved graphics
+ Better "mining", and now you can put the mined minerals to use.
Negative:
---- Severely pruned customization of... almost everything, from skills to weapons
Not sure about:
+- The thermal clips, instead of overheating and unlimited ammo. I hate running around picking up dropped thermal clips, but on the other hand, I tend to use a wider variety of weapons in ME2 than I did in ME1 - being forced to do so because of running out of "ammo". I'm not sure whether the balance goes to negative or positive.
+- The "mission" format. Feels somewhat out of place in an RPG.
Overall, the writing is good, as usual with Bioware, I like the companions, and I'm enjoying playing the game. Isn't my favorite Bioware game, but a very good one nevertheless.