My n900 is quite rock solid.
The only issue I have is that after 4/5 days of uptime it does not connect to wifi automatically, I have to reboot.
Some time it seems to be connected, anyway my voip accounts are marked with problems (red icon with exclamation mark), reboot fix that again.

Anyway about 2 months ago it was very unstable, but I was in a very experimental phase and installed a custom built kernel with nat, a lot of packages from extra-testing/devel, and some .deb from debian armel.

When PR 1.1.1 was released I reflashed the device and restored from a backup without the list of installed apps, formatted mmc too, and was very carefull in installing new apps, tracking them on a text file (repository, or other sources), to remove them in a clean way, using apt purge instead of clean, and so on.

So I'm not sure if it was PR 1.1.1 to make my N900 reliable or my care

Niko