I haven't had any problems with "Upgrades" at all.. and some on boxes the "Upgraded" from Jaguar -> Panther -> Tiger.
That being said, what I will do, and encourage you to do the same:
Get an external HD.
Use SuperDuper to clone your boot device to the external HD.
Confirm that it is a bootable, functional, full backup.
Wipe the boot drive and install Leopard.
During the initial account creation process, you will be asked if you want to import data from another drive.
Guess what? You do!
Use your bootable, full backup, to import your previous data into your new Leopard install.
If Leopard isn't working out for you or you want to wait until the first patch, you can always go back to your bootable, full backup.
The best of all worlds.
I have spoken.
That being said, what I will do, and encourage you to do the same:
Get an external HD.
Use SuperDuper to clone your boot device to the external HD.
Confirm that it is a bootable, functional, full backup.
Wipe the boot drive and install Leopard.
During the initial account creation process, you will be asked if you want to import data from another drive.
Guess what? You do!
Use your bootable, full backup, to import your previous data into your new Leopard install.
If Leopard isn't working out for you or you want to wait until the first patch, you can always go back to your bootable, full backup.
The best of all worlds.
I have spoken.