My copy arrived yesterday and so far, my impression is that they've totally blown the previous two games out of the water. Not just the big stuff, but the little details too. The Citadel for example feels much more like a living place. For one thing, people can move around! Seriously. It makes a *huge* difference.
The only things that's bothering me is a few broken quests I've run into. Both on the Citadel and both involving major returning characters from ME2. A quick look at the bioware forum shows that I'm not alone and this is happening on all platforms Hopefully a fix will turn up soon! There's also the issues of sidequests becoming unavailable if you don't complete them before certain points. I was actually aware of this already, but still managed to get caught out.
So, just in case anyone else doesn't know yet; make sure you do *all* the little sidequests before you do a mission marked "priority" because they progress the main plot and things tend to change after you do them. Can't really be more specific without spoilers!