I thought I would pipe in as a SGS user (seems no one in this Desire sub thread has used one from experience).
I have played with a Desire and if it was available on my carrier I probably would have got it instead - only because I like the Sense UI better.
I was choosing between an "i word" 4 and an SGS coming from a Iphone 3G, the original Samsung Galaxy and a Blackberry 9700 so I have used all types of phones and OS.
I made a split decision to get the SGS as they are selling quite fast here in Australia - I didn't want to miss out. As soon as I held it in my hand I was dumbfounded how light it was. In a good way! Yes it looks like the iPhone 3G and I like that, because people will think that's what I have and then BAM, no sir, it's something different!
I don't think it's "cheap" looking, but I do think it's a smudge magnet. But I don't know a touchphone that isn't. It fits nicely in the hand and despite it's size, it's not annoying once you've got a hold of it.
Then the screen turned on, again, I was in awe of it's fabulous colours. I keep mine on the lowest display brightness and it's still amazing. Coming from the original SG, it wasn't as "flowing" as an iPhone which gave me the $hi%s and ended up putting it back in its box a month later. The multi-touch screen on the SGS is sensitive and comfortable, like an old friend. Text is clear and precise.
What does annoy me about this phone is obviously the big chance of it not being able to get Froyo. It does not have a camera flash or LED notification (which I relied on, on my BB). The speaker phone is quiet and if it's resting on something it muffles the speakers on the back. So far that's all I can complain about.
It doesn't lag, everything seems to respond very well. Phone quality is excellent. I LOVE SWYPE (no idea if that is on the desire) but it definitely is the fastest way I've been able to type on any phone yet and I don't get the cramps in my fingers from a physical qwerty keyboard like the BB when doing a big long email.
Overall - I thought I never would be able to change from an iPhone and be satisfied. I have wanted to, especially with my love for Android, but always felt there was something that just didn't cut it, until now... I love my SGS, and I am surprised I do. But like I said, if the Desire was on my carrier, I probably would of gone for that because I was VERY skeptical about the SGS. At the end of the day, specs aside, it comes down to the little personal preferences that make a happy phone user.