I don't know about ME2 (I own it but I haven't gotten to it yet) but I'm replaying ME1. I've made a couple of decisions differently, this time around. I've sided with people I didn't side with before and I'm playing a little bit shadier, this time around. I can honestly say that it's a very different game, this time around. Mys options for choices are different and the missions I get are also different. I suspect that, if my decisions carry over into ME2, it's going to be a different game than without.
My only regret is that I'm playing as female Shephard instead of male Shephard.