I have the N900 and have done for over a month now - it's absolutely fantastic!

Beats any of the above! IMO

Iphone - better camera and faster - 600mhz processor
HTC HD2 - Great phone hardware wise but as someome said above the OS lets it down fantasticaly
Blackberry - good phone for business but not for me - didn't like it at all - Seemed tto businessy - but I suppose that depends on which one?

The N900 has by far the best web browsing interface i've ever seen on a phone, probably because it's a internet tablet that can do phone stuff as opposed to a phone that can do internet stuff.

All apps are free! (IIRC might not be right on this one i'll have to check)
Camera is ok but my last phone camera was 8mg so maybe my standards are too high?
Full qwerty keyboard is great and I can now text without looking at the keyboard (like the good old days!!)
Multitasking is fantastic, phone has no problem with having several applications open at once

I love it and i'm sure I still will in another 12 months!