I previously used Sony readers and the 505 was really flaky, I have frequent reboots with mine and it wipes collection and bookmarks when it happens, rare but it does happen. This annoys me far more than my Kindle, plus Sony just don't seem to care about updates for their readers.
As for the Kindle 3, mine has frozen and rebooted a few times......however, has it made me wish I hadn't gotten it?
NO WAY, I LOVE THIS THING !!!!!
I really don't care about the odd freeze, it can be reset and I don't lose anything when it happens (as I back everything up with syncs)
I know Amazon are currently testing an update so it will be fixed soon, if there are still issues then it can be replaced, as Amazon have the best after sales care.
The Kindle is so much fun to use, don't be put off, you will love it