ICS is great. The GN not so much. I think ICS on SGS2 would be amazing though. The GN is too big, too unwieldy. It's just huge in my hand, and I'm very comfortable using the Sensation (my current phone) so it's not like I'm used to something like an iPhone which is quite small.
I tried to fit it into my trouser pocket as well and that proved difficult as well because of the size.
ICS is really great though, Google have really cracked it this time. Stock ICS is better than any other phone experience including iOS (which I thought had the edge until now). I used it for about 10-20 minutes and there was no slowdown or lag at all. Transitions were buttery smooth and animations within the menus were just brilliant. Google have done a great job, and if any blog or website complains about Google not having smooth transitions they are lying to themselves and their readers out of pure spite or fanboyism. The SGS2 had poor transitions and my Sensation chugs sometimes but in the whole time I used the GN it was smooth, even with music playing, surfing a content heavy blog and downloading a trailer using tTorrent.
ICS will be a big success in pushing Android to the next level. The Galaxy Nexus, not so much.