I was in Vodafone last Friday faced with the same decision (Desire, N1 or Galaxy S) and I chose the Nexus One simply because it is "the" Google Phone and gets all of the latest updates straight away and isn't tainted with 3rd party UI's (Sense & whatever Samsung put on the Galaxy S) - it just feels more "pure" if you know what I mean.
Anyway, a quick run down on each of the phones from first impressions:
Galaxy S: Fantastic screen, very vibrant and bright. Poor UI design and at times it's choppy going between windows and scrolling through the apps. The build quality is lacking compared to the Desire and Nexus One, it just feels too plasticy and toy-like.
Desire: Nice screen, not up to the same standard as the Galaxy S. Not keen on Sense UI but it's 100% better than Samsungs UI. No apparent slowdown and the build quality is really good, it feels like a solid bit of kit in your hands.
Nexus One: Same screen (I think, it looks the same) as the Desire. Running Froyo (2.2) the UI is brilliant, very responsive and no apparent slowdown which I have noticed even after nearly a week with the device. The build quality is the same as the Desire, very well built and feels great in hand. I didn't try the Desire or Galaxy's camera but the Nexus One's is really quite good, so is the video quality - It had extensive use at last weekends Howduino event in Newcastle and I was very impressed.
The only bad thing I have to say about the Nexus One is that you run the risk of getting one of the phones with the WiFi Sleep Policy issue; apparently Vodafone, HTC and Google are working together to fix this at the moment though. There is also an app called WiFi Fixer which solves the problem for most people in the meantime. Overall I
love the Nexus One, it's by far the best phone I've ever had
Hope that helps
