being one of the lucky 300, i have heard the question "so, what's it like then" a lot.

My answer is that it is the best all in one mobile device on the market at the moment, but to think of it as a mobile Linux computer with a built in phone, just like the iPhone is a big iPod with a web-browser and a phone, and S60 phones are phones with built in computers.

It is NOT best at any one thing, but it scores highly in EVERY catogory, so ends up as the winner for me:

• camera resolution/quality

the N86 has higher resolution, the N83 has Zenon flash, but it is WAY better than the iPhones camera.
Hopefully some of the code from the S60 branches can be ported without too much effort to help out here

• maps & GPS

the maps implementation on the N900 is brand new (it matches Ovi Maps via web very closely, and has lots of Javascript driving it) so gets a "meh", the GPS can struggle to get an intial lock without AGPS (i.e needs to send some data over cellular network), but the maps can be stored locally on the phone so no huge data bill when roaming (like Nokia/Ovi Maps on S60 phones)

• audio playback

very good speakers, but the software UI is unique (not sure if the iPod UI is better as I am not that big of a music fan). The included headphones are good, the current Nokia in-ear type with the read tag.
Nice touch, is that the indicator shows if they are detected as just regular headphones or also a microphone.
Gpodder has caused me to discover the world of podcasts, so it is becoming more important

• physical size

no denying that compared to current phones it is physically large, but it's as small as you can get while still being able to make use of the 800x480 resolution

• web

The inbuilt browser (microB) is good, but like everything struggles with Javscript or Flash heavy sites (causes high CPU which in turn increases battery drain)
It is leagues ahead of the S60 browser and there is Fennec coming as well
I have only briefly played with MobileSafari so can't really comment.
Major drawback is lack of copy from web pages

• mobile computer

This is where the N900 SMASHES the competition, iPhone is closed, Android requires Java(shudder), S60/Symbian is a phone first & computer second. Shame the Man pages are not included (man man on terminal returns command not found )

• UI

Takes a little getting used to, but the intro video does an excelent job of that. A bit more complex than iPhone, but then it's also more flexible (and in my book that is a good thing!)