If you haven't upgraded yet - don't! I bought my first Apple computer in 1978 - the original Apple ][ - and the obvious quality of the products has kept me coming back for more. Good thing the Leopard upgrade wasn't my first experience of Apple quality.
10.5.1 isn't it, either. I had great hopes that the first dot release of 10.5 would fix most of the problems. Sadly, that is not the case.
Most Leopard installs are trouble-free. But if you rely on your Mac to make a living, you have to ask yourself if it is worth the risk to upgrade for nifty but non-essential features. I haven't upgraded my backup Mac and I won't until Leopard is stable.
Tales of woe I use my Mac for writing, research, Skype, web site creation, video production and much more. Since installing Leopard, I've had to re-install OS X 3 times, Final Cut Studio 3 times (it is on 8 DVDs, so it takes a while) and boot up into the Unix command line twice to re-create the user database.
In addition there are numerous other inconveniences, such as Keychain problems, out of date third party apps, broken application services and more. They slow me down.
But the biggest problem is not knowing if the system will start up reliably. I don't need that problem from my Mac. With OS 10.4 I didn't.
The Storage Bits take AFAIK, the vast majority of people don't have problems with Leopard. But if you are a power user who needs a reliable Mac, the problems are common enough to wait for Apple to resolve them.
I suspect that Steve's commitment to ship Leopard in October led to some corner-cutting in development. Once released developers also have to deal with the bugs customers report.
A few more months of development bake time would have brought Leopard up to Apple's normal high standards. Steve, next time let the developers, not the calendar, tell you when the release is ready.
Comments welcome, of course.