Apple's Installation and Setup support forum for 10.5 (Leopard) now rings with the cries of the damned. The inevitable "wait for 10.5.1?" posts are appearing, but Apple is killing them within an hour of posting. (I tested this by commenting on one of them. It was gone an hour later).
This is not at all surprising. We see it even with only minor updates, much less a major OS transition. Apple is very secretive, and secrecy is the enemy of quality. Sure, the Vista update is much worse, but Apple controls the hardware and Microsoft doesn't. Given control of hardware Apple should have much smoother OS transitions, but the secrecy is lethal.
Most people will do fine. A number will lose all their data. They ought to scream, but I don't think Apple will change its ways.
Overall 10.5 looks to be a very promising update. Unless you're buying a new machine, an update like this is strictly for the foolish, the prepared geek (two backups, different methods, one a bootable image for quick restore), and those blessed by the Gods of Apple (Andrew).
I expect to be pretty happy with 10.5.1 and very happy with 10.5.4.
I wouldn't install 10.5.0 if you paid me.