It's rebooted on me twice that I know of, both times shortly after pulling it off the charger in the morning. (It's possible it might have rebooted more times overnight, without my knowledge, as I generally do not turn off my phone.)
Due to the low number of failures, I had assumed it to be due to a faulty app or two somewhere.
I've also occasionally had the phone freeze on me a few times on GB, but all of those times I had a lot of apps running concurrently (tweetdeck, browser running flash, lastfm, and music being streamed through the player,) so having the phone choke wasn't entirely unexpected, even if it was a little annoying. Sometimes the freeze clears out after a few seconds, and sometimes it takes a battery pull, but it's not something that'll put me off using my Droid X.