Brief first impressions...
Very, very impressed. I'm still fiddling around with the various aspects of TouchWiz which is much better than it was on the original Galaxy S, but its certainly going to be removed at some point in future
The screen is just superb, its not quite as sharp as the Retina display on the iPhone but the difference is hardly noticeable in general use. Everything else about it is better.
The camera is excellent, this is the first phone camera that I've found is clearly better than the one on the iPhone 4. The video recording appears better yet, I haven't tested it out properly though just yet
The build quality seems absolutely fine, the phone is very light as I was expecting and it feels totally solid. It doesn't feel as premium as the iPhone but it certainly doesn't feel cheap.
Speed-wise, its very snappy and flash is almost 100% smooth in the browser, the first time I've experienced that on an Android phone, impressive stuff.