I'm running a VMWare image of 10.6 (Snow Leopard) right now. I didn't set it up, but it does indeed run. It's not perfect. It had problems running Safari 4, but now runs Safari 5 fine. I use it for browser testing. That being said Flash video crashes it hard, HTML5 video crashes it gently (like caressing a child's face with a butcher knife).
I have a pretty capable machine (Core i7 920, 6GB, GTX260 video card), but speed wise it is quite usable and I run all sorts of osx-only apps on it (like the omni suite) every day.
So.. i guess like 7.5 out of 10? Buggy, but really getting close.