Coming from an iPhone, using the EVO keyboard is wonderful. I do like having an external keyboard for games, doing SSH/console stuff, and the faster navigation (start typing to search, spacebar to scroll down).
However after my very poor experience with my Motorola Backflip being locked down and not getting firmware upgrades, I'm really nervous to use non-HTC phones. I know it takes HTC a long time to upgrade firmware, but so far they have done better than moto has with blur phones. I flip back and forth on if I like Sense better than default Android, but I certainly like blur and touchwiz worse than default. Plus I think Samsung locks down their phones more.
The camera is probably actually a win for Samsung. The 5MP on the backflip was pretty much just as good as the EVO, and Samsung is a real camera company. Also I think video on the Galaxy S is supposed to be better than the EVO, which kinda stinks.
The other issue is the screen. I have a very hard time seeing my EVO screen in sunlight, and it sounds like the AMOLED screens are even worse. Samsung claims the Super AMOLED is great, but I think that remains to be seen. If the Galaxy comes out and reviews say the screen is better in direct sunlight than the EVO, and it gets as hacked as the EVO has been, I'm going to want one. Otherwise, I'm very happy with the EVO.