I'll just say this:

I've used my N900 heavily for 6 months now. Done all the updates OTA. Done some mods like overclocking, changing the boot-up video, etc. Installed a bunch of stuff and have testing always enabled. But I've been pretty careful what I install from the devel repo and leave it disabled by default.

After all that, I've never reflashed and the phone runs perfectly. My uptime is measured in weeks. I don't expect to ever reflash the phone as long as I own it, unless I do something that requires it like installing a non-upgradable version of the OS (MeeGo sometime in the far future?) or I just get suddenly stupid and manage to trash it.

Maemo is linux and I expected it to be as solid as a rock. In over 10 years running desktop linux I've run the same installs for years. Honestly, I've never reinstalled any linux OS because I trashed it beyond repair. I've gotten fairly stupid a few times but found a fix every time.