While I cannot speak from experience for Leopard, I can confidently say that Tiger works great on my hardware. I have been using it as my primary OS for almost 3 months now, and I use it heavily for Photoshop and the like. The OS has been very stable and almost every feature works. And yes, I am using the latest Tiger version - 10.4.11.
It is possible to install Leopard on a normal PC hardware too. But it's not gonna be as easy as installing Tiger for several reasons:
1) Leopard has been out hardly 2 months so most installation methods are very crude - command line and all. With Tiger I hardly had to enter the command line mode
2) Leopard will most probably not support older hardware out of the box. Because it was not designed to do so.
3) Leopard will have frequent updates from apple because its still in its nascent stage, and installing updates directly from Apple is a pain. This is true for Tiger too. So even if you do install Leopard, the possibility of it breaking any time is very much there.
4) Lots of beta versions of Leopard floating around online. Its hard to get hold of a proper working copy, especially since the green demon is down.
There are few more reasons as to why a Leopard install on a PC is difficult, but bottom line is - yes, its possible. But be ready to get your hands dirty.
