Ok, let's see who's going to hit me on the head.

Imagine that i want mobile internet, a good PIM that syncs with Outlook/Gmail, an outliner.
Imagine i do nothing with music or video (yes, i'm the only person that knows that internet exists and doesn't have MP3's on his PC).
Imagine that i just want things to work and i love being root, but not on a device that i'm relying on for work and personal use.

Apart from losing a fairly decent browser, what do i lose when i switch from my N800 to the BlackBerry?

Ton.

You pretty much hit the nail on the head. You will get excellent synchronization and push email. Unfortunately, you will also get a very serious downgrade in browser capability. The Java based browser, on the Blackberry, is even more limited than the one on other smart phone platforms like Palm and WindowsMobile.

If you sync with an exchange server, and you've got push mail on it, then look at the wm smartphone devices like the Motorola Q9, or the HTC Excalibur. You'll get excellent syncing, a cr4p browser, gmail over pop3, and an mp3 player for if you ever discover them

I used to run an O2 XDA2 then HTC Tytn until I discovered the n800. The Tytn is a fantastic device for mail, and the Q9 is nice for mail/sms purposes.

The big advantage over the blackberry's is that you don't have to subscribe to their over-priced services, on top of your regular line rental.

The Blackberry is best used with Exchange and a BES server. With Outlook and Desktop Redirector (comes with Blackberry) you lose a few features, but it's still good as long as your computer is always on. And having a keyboard again is so nice. My killer app, besides email/calendar, on the Blackberry was Google Maps. Didn't have GPS, but it was still a life saver many times.