I'd be interested to hear more, Ed, since you've used both.
Looking at images people have posted from both lenses, but no side-by-side comparisons of the same subject, my impression is that the 2.0 is going to be sharper handheld, thanks to IS, maybe on a tripod as well, but it's hard to tell that without a head-to-head test of a static subject. Out-of-focus rendering looks consistently smoother to me with the 1.8, and not because it's a faster lens, but because it's a different optical design. And 1/3 stop is 1/3 stop, and generally I'd rather have a faster lens on a tripod than a slower lens handheld with IS.
1.8's seem to have a serious issue with the focus mechanism wearing out over time, which is a concern, and it seems like the 2.0 is better sealed. The 2.0 is also lighter, despite having more elements and IS. I don't know if the later version has better autofocus speed, but in general I don't use autofocus, so it's not a factor.
Does this sound about right to you, and are there other issues you've noticed?