I have upgraded 5 Macs already, and they are all working fine. I had one post installation problem on my wife's MacBook due to Application Enhancer not being compatible with Leopard. I found on the Apple forums that this was a problem and to remove the files, rebooted, and all was fine.
I do not see any slowness, and I really like the way it looks. Time Machine is really great. A little hint there, make sure Spotlight is configured NOT to index your backup drive or it will take forever to backup.
Macs upgraded are MacBook CD 2.0GHz, MacBook C2D 2.16GHz, iMac C2D 17" 2.16GHz, Mac Mini PPC 1.67GHz, and PowerBook G4 1.5GHz.
Just wish my 800MHz iMac and Dual 450MHz PowerMac could be upgraded.