FHF  SuperDuper! just saved my bacon. After finally deciding to get a little more modern OS, today I spent hours upgrading my G4 PowerBook from 10.4.11 to 10.5.6. After doing the install and then all the updates I finally got ready to use my PB. Opened Safari, realized that it was v.3.2 instead of the v.4 beta I had been using prior to the update. Went to the Apple website, tried to download the v.4 beta. No go. Would not download. Hm, that's strange. Ok, let's try Mail. All right, all my mailboxes are there, but where are the messages? All. Gone. I can see that there are lots of messages in User>Library>Mail>Mailboxes, but damned if Mail can see any of them. 'Rebuild' a mailbox. Nada.

Searched for 'Leopard...upgrade...lost...mailbox', found a few other people who had had the same problem, but no solutions. Crap.

Screw this. A new flashy OS is not worth this kind of headache.

Restarted from my SuperDuper! backup volume, told it to 'restore' and in about ten minutes I was back where I had started this morning. Whew! That's gotta be the best $28 I've spent in a long time.   
(Version 2.5)

FHF  This version of Flip4Mac WMV Player brought my Aluminum PowerBook to its knees. For weeks my computer randomly crashed at least once a day. In addition, it would often refuse to sleep, or had a dark screen (and was unresponsive) when awakened from sleep, or went dead during sleep. Occasionally got the 'gray screen of death' ("This computer must be restarted"). And when it was working, internet access was very slow ("contacting xxxx"). Extremely frustrating.

Re-installed OS 10.4.11, ran disk permissions, DiskWarrior, nothing fixed the problem. Started removing any system modifiers one by one: Quickeys, SafariBlock, EarthDesk, Perian. No better. Stopped using my wireless USB mouse. No better. Finally removed the very last non-stock item: WMV Player. Bingo. I can now put the computer to sleep as often as I want, it wakes normally, internet access is satisfyingly zippy. My PowerBook is born again, and has been working fine for several days.

One by one I re-installed Quickeys, SafariBlock, and my wireless mouse. No problems whatsoever. I'm not about to re-install WMV Player to relive my misery; there's no doubt in my mind that it was the source of my difficulties.   
(Version 2.2.0.49)