I've finished Mass Effect 2, some 45 hours of game saves later. What a great game! I thoroughly enjoyed it start to finish, my attention never wandered. I even enjoyed doing all the NPC side missions for their loyalty. Mostly because the character stories are great, but even with the characters I don't care about it was fun to go somewhere new and solve problems by shooting things.
Like I noted before, Mass Effect 2 does a great job on the gameplay. I have some nits, particularly the way most powers are useless against enemies with shields or armor, but in general the combat mechanics and levelling and the like were really fun.
But the best thing about this game, like all Bioware games, is the narrative experience. The interactive movie feel. I'm particularly impressed with how Mass Effect 2 really nailed immersion for me. That's me, up there, Commander Shepard. That's my armor, that's my voice, that's my identity through two of these games now. Normally playing games like this I totally ignore or skip cutscenes and dialog. Not so with ME2, something about seeing myself in those scenes, my personalized Shepard with my voice, made them really work for me. The voice thing in particular surprised me; voiced main characters are controversial because for some gamers it breaks immersion. Not for me, that voice actor was me. A heroic, blond me in orange armor.
The side plots and characters were the most enjoyable part. The grand space opera of the main story was OK by me, but I have to agree with Shamus' criticism (1 2 3) that the whole setup of the story was pretty lame. As much as I enjoyed my decisions from Mass Effect 1 carrying over to the new game, having you be destroyed and rebuilt is pretty weak sauce. And I agree with Shamus that the main plot doesn't make much sense. It just didn't bother me much. I guess my standards aren't very high. Still, it'd be nice to see an epic game have the plot consistency of at least Star Wars. That's not a very high bar.