The general tone of this conversation makes me inclined to chime in that the first post in this thread should have read:
"1. If you don't know how to back up your files and database
2. If you don't know how to restore your files and database
3. If you don't want to upgrade to 2.6
Don't do it."
Self hosting your WordPress is a challenge and you need to educate yourself in managing your software and installation.
Are there problems? Sure. That's the way it is; the software development is driven by the best efforts of volunteers and unfortunately not all test cases get caught.
Is just advising people to not upgrade really helpful? The only mention of security I can see in the release announcement concerned better SSL support (very cool) and "A number of proactive security enhancements, including cookies and database interactions". No mention of "upgrade or lose everything".
So if you want to take the plunge and upgrade, then backup, get ready to restore, and enjoy if it works out. If not then keep enjoying 2.5.1.
BTW I did 3 upgrades without problems. The new plugin management page alone is worth it for me.