I have a N900 'phone. I love it - basically it is Linux in your pocket, with proper web browsing and a telephone. I think anyone who works with computers should consider looking into buying one.
However, as a "business" 'phone it sucks. The basic requirements of a business 'phone are:
1) Push e-mail with integrated spellcheck (you don't want typos in the e-mail back to the boss or important client)
2) Easy to use calendar (which syncs with desktop data)
3) Easy to use address book (which syncs with desktop data)
4) Easy to use 'phone (one-handed dialing, quick-lookup in address book, etc.)
The N900 does not compete well with either the Blackberry here, and to a lesser degree the iPhone. Fix those four points and the N900 becomes a serious contender for a business phone (since the business can install any damn application on the device).
Also, the Ovi Maps application for the N900 is currently underpar - hopefully this will improve later this year.
I really hope Maemo takes off. This is the platform for Slashdot readers. The N900 is a great device to tinker with.