Don't see the appeal of Tali AT ALL. People kept saying how much they wanted her to be a love interest in the first game, but I just didn't get it. She's a nice enough character and all, but for a love interest? No thanks.
Anyway, I definitely cheated on Liara with Miranda. Here I thought she'd be all worried and thankful that I was alive when we met again, and all you get is one kiss. Then she's all cold and asking for favors. Well, f*** that, then.
Actually, that's a bit of a nitpick I have with the game. The love interest from the first game, whoever it is, got so shafted here that you barely have any reason to remain faithful. Not to mention the fact that ME2's LIs are much more interesting (except for Tali).
And that's one of the best things about the game: the characters are all incredible. Even Jack, who I thought I would hate, ended up being incredibly sympathetic. I do miss Wrex though. He's way better than Grunt.
EDIT: Just to add that Thane f***ing rules all.