The n900's hardware is currently unbeatable. As for software (OS, applications, etc) it's in a constant state of flux. Things are improving by the day. As of 2010/04/11,

What it's excellent at:

- Web browsing/flash
- Skype, IM messaging
- Multimedia (lots of codecs)
- Ourdoor (video)camera
- Unix functionality (you may find a use in the future)
- Exchange sync and contact organization
- Document editing (full openoffice support)
- Connectivity (bluetooth / wifi / USB)
- Stability and performance (especially when overclocking, which it loves!)
- Navigation (with Sygic navigator)
- Multitasking

What it's decent at:

- Productivity (email client is OK, but not great; calendar is weak)
- Phone functionality (phone works fine, but lacks some call management features)
- Indoor photos (low light)
- Gaming

What it's terrible at:

- Battery life (I average ~24h per charge, meaning a charge every night)
- GSM frequency support (I forget which bands it lacks, but it does not support them all, so 3G can be limited)
- Simplicity. Some see this as a positive: The interface is very powerful and configurable, and pulls no punches. But, for those who want something braindead simple, it's not the right choice.

Personally, it happens to be the best technology thing I've ever owned. Like others have said, if you're looking for a pocket computer that happens to have a GSM radio/telephony, this is the best device money can buy. If you're looking for a phone with a great web browser, it's competitive. If you're looking for a business tool or fashion accessory, Blackberry/iPhone's a better choice.

My $0.02.