The only time I have seen a data loss like that ended up being a hard drive failure. After it crashed, I ran the disk utility and it failed the SMART hard drive status test. It rebooted every time and ran normally. Luckily, it was not my Mac. I personally have never had a catastrophic failure or data loss. Since 2004, I have only had one crash and it recovered flawlessly.
As for OSX 10.5, it's been a bit buggy but manageable.
It's had a few Safari, Java and other quirks, but the worst so far has been removing the administrator privilege on my MacBook. I could not run scripts, software updates or any other program that required my keychain access to my user account. After setting up a root admin password and a few tweaks it was as good as new, but rather annoying to say the least. In your case, you sound like a perfect candidate to run time machine until you, or Apple works out the bugs. At least you can use your external and the migration assistant to restore your system. I work on Microsoft junk all day at work, so I am biased to the fact that I rarely have to come home and work on my Macs. It's still, in my opinion my primary operating system regardless of the small bugs.
I find myself agitated to have to wait for a Windows based OS to boot, run slowly and just plain stink. People at work ask me why I tap my fingers when rebooting their computers. My simple answer is " It's not a Mac and I don't have all day". You can try Leopard support, but I am sure they are getting blasted with calls and error reports. Try running a hardware test and disk utility to be sure, but in the meantime, keep the faith.
A free 15 day trial of Mac Pilot is available from
www.koingo.com. Under Tools, it will help you run scripts, verify disks, preference files, clean up and erase scratch, junk, Icon and unused preference files. I also run update prebinding, Rebuld prebinding etc. Under Disks, I can run repair permissions, verify permissions and verify disk. It's very helpful tool and if your luck, just may assist in clearing up some confused preferences or files.