Ranked in these QuestionsQuestion Ranking
Pro Has an active community developing applications and OS updates
The N900 has an active community, developing applications and OS updates and updating packages for the device when needed. You won't get facebook for it, because facebook itself cut off the leash to the older phone OS's, but if you want that you shouldn't want this device either unless you can live with the browser version. It's for those that want a pocketable PC.
Pro Has a physical keyboard
The N900 has a physical keyboard that can be opened by sliding the screen up or down from the side.
Pro Very compatible with Linux
The N900 comes with Maemo 5 installed which is a Linux distribution. There are also some Debian versions that work on this phone.
The N900 was first released in 2009 and it shows. The hardware is outdated (256MB RAM for example) and there are no more software updates being released.