I'm about eight hours into this, and I'm totally sucked in. As I have a PS3, I've only previously played ME2, and while I overall loved that game, I was semi-disappointed that the decisions I made throughout the game didn't have a more tangible effect on the story (no pun intended) and more consequences for the characters than they did. This is where I wished I could play ME1, to see if it was any different in that respect, as even with regards to ME2, that was accused of being dumbed down from ME1.
However, I feel this have already been improved upon in ME3 in the relatively short time I've been playing it. Your decisions seem to affect the outcome of immediate events pretty directly (and at times shockingly), not to mention the long term consequences that I'm sure they'll have on the story. In one particular mission I've played, it makes it pretty clear at the start that you'll have a big decision to make, so as you're playing through the mission you get time to think about it, and at one point even get a chance to knock it on the head, but the time comes when you have a choice to make and the effect it has is pretty devastating. You feel genuine guilt and and keep thinking 'what if' in terms of doing things differently. As much as I loved ME2, I didn't really feel that at all with it. That alone makes ME3 a winner for me, as the whole decision making thing was what I was looking forward to the most when I first played ME2 after hearing how awesome that whole thing is in ME1 and even going back to The Knights of the Old Republic, and in that sense, ME2 didn't quite measure up for me, but ME3 so far seems to be making up for that.