My friend got the Desire as well and my 2 cents:

- Plenty of apps on Android. But didn't find that many extra special apps compared to N900. I searched 100 most popular, top 100 must have apps etc etc and I don't see much useful for me. Certainly not tons better than N900 imo. I much prefer my geeky desktop apps such as openssh, truecrypt, midnight commander etc..

- It can't play xvid videos? That's one of my main uses for my N900. So this is real bad news. I hate converting vids. Been doing it with my PSP for a long time.

- Desire is obviously not comparable to N900 hardware wise. It has a worse screen, worse camera, worse video recording, no active lense cover, no camera shutter button, weaker flash, no hw keyboard, no stand, no fm transmitter, no built-in 32GB memory in addition to MicroSD slot, no TV out, no stereo speakers, no resistive screen which allows me to use finger nails or any hard object I happen to have in my hand at the time....... Sheesh. That's a lot.

Well, Desire is a more "smart phone" than the N900 is.
So for those who wants that, it's a great sleek device.
For me, I wouldn't swap my N900 for it that's for sure.
I see nothing but downsides to it. Dunno. Just do.