I've had the Galaxy S for just over a week now, upgrading from the HTC Hero. I can honestly say it's by far the best handset I've ever owned.
The screen is this phone's USP and, despite it not quite reaching the same res as the iPhone 4, the actual image quality is much better thanks to the 4" Super AMOLED display. Compare it side by side with an iPhone to see what I mean.
Low light snaps are surprisingly good considering the lack of any sort of flash or photo light, and video recording and playback are superb.
I am a big fan of HTC's Sense UI, so I was a little unsure about switching to TouchWiz. However, I find that Samsung's interface works really well. You obviously don't have the excellent HTC widgets available to you, but there are some very nice ones provided by Samsung with hopefully more to come. Also remember that you have the option of installing a different launcher if you don't like TouchWiz - LauncherPro (available in the Market) is very good.
One thing that never gets mentioned is that Samsung provide the excellent Vlingo speech recognition and reader software free of charge (via their own app store). This is usually $10 in the Android Market. You can send texts and emails, make calls, search the web, find places on a map, etc. all by voice control - and it works surprisingly well. You can also have it read texts and emails to you as they arrive.
Overall, I'd recommend this handset to anyone looking for a new Android phone.