I ran a VMWare image of OS X a few years ago. I don't know what it's like now, but back then the lack of an OpenGL driver was the deal breaker. OS X's graphic shell uses Quartz Extreme on top of OpenGL, without GPU acceleration it was virtually unusable.
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.
