Meh, Garrus's stuff is quite good. Anderson is solid. Femshep is much better than Broshep. She's often melodramatic and there are plenty of those cheesy lines but there are also some very well delivered ones. Same goes for a lot of party characters. There's plenty of good voice acting, and some bad. Totally needs some Nathan Fillion though.
And yeah, the writing is not great (Tricia Helfer does a pretty damned good job with some pretty awkward material) - it is however better than most video games and is supplemented by the plot and the universe. I wouldn't say it's "run of the mill" sci fi, it's a pretty damned compelling concept. The series is actually the first one on this generation of platform, I think, to really effectively give you a sense of "large stakes" in the conflict. The antagonists are introduced effectively and are believably capable of destroying everything because they've already done it. The alien races are diverse, culturally and historically interesting, pretty well developed all things considered, and have tangible political tension that's actually understandable. I think it's totally forgivable, but then the RPG's I grew up with were mostly JRPGs and... well, yknow. Exhibit: Xenosaga.
I see the Mass Effect series as the biggest step ever in the path towards video games becoming a viable storytelling method. As far as emotionally compelling, I actually did find the ending part of the shroud mission, played as a renegade, pretty damned well done - granted, still cheesy, but... well stop being a killjoy.