I enjoyed ME2 greatly, both as a standalone product, and as a continuation of ME1's storyline. Given the narrative problem of "reducing" Shepard from the ME1 endgame, that's no small feat. They handled it remarkably well, IMO.
The plot of ME2 is simpler than ME1, and more linear (though gameplay is not linear...you can do most missions in any order, though there are some 'gotchas' in the plot missions). What makes ME2 extraordinary isn't the story of it, which has few surprises, but the degree to which it adds onto the setting and mythology of the series. It even addresses some of the Big Questions from the first game; about Reapers and their methods and motives.
I also like how they show, rather than tell, how the Terminus systems are radically different than Council space. There are no big governments here, no enforced civility, no big fleet backing up the Council's soft talk. Even relatively civilized worlds like Illium have a sort of lawless, "everyone for themselves" feel to them.