I've been running Ubuntu Dapper Drake (the development release) for quite a while now... well, almost six months. Before that I was running Ubuntu Breezy Badger when it was the development release. So I've not had a reinstall of my main machine in almost a year... never a good thing when each day I was averaging between 60-120MB of "updates".
Lately I've found that X has been locking up, the screensaver seems to cause my session to die, and so on. There are three possibilities here: 1) it's a big nasty bug; 2) my GNOME user profile is boned; or 3) the install is dead.
The easiest thing to fix is the install... I've got a whole disk for /home so I simply reinstall Ubuntu Dapper Drake Flight 5 over my current install and then applied the latest updates. If the problem occurs again then I'll create a new GNOME profile... this takes a bit more time and effort but still isn't too much hassle. If it still occurs that probably means it's a big nasty bug, that I'll have to work on tracking down to get it fixed.
Still, I'm pretty happy with the runtime on my development desktop... it's been more stable than the average "stable" Windows XP install, so that's one thing ;)