Ditched my Desire for this fella last week and couldn't be happier.
Thought I'd miss HTC Sense, but am now rediscovering all the cool little apps (and new ones) to do things better than Sense does. The lack of SD expansion doesn't bother me to be honest, and given that this will likely be Google's reference platform for a bit, I wouldn't be surprised to see it happening on other phones.
Whilst I can see the tint some people are mentioning, that's something that doesn't concern me either.
I'd say in the 4 days I've had the Nexus S, I've got more done and had more fun than with the Desire in months.... whether that's the same months down the line from now, it remains to be seen, but for now I'm well chuffed.