Translating Emulation

I'm intrigued by the Rosetta technology. It is a means to run PowerPC applications almost full speed on MacTel. It's from a company called Transitive, a Silicon Valley startup formed from researchers from Manchester University, England.

The way it accomplises this is by partially translating a bit of code, running it and using memory caching to minimize the amount of times it does this. It also tries to optimizie anything it can to increase performance.

This sounds very similar to the way that SUN speeds up Java using Hotspot ! :)

What I don't understand, is if this technology is so good, I would expect in turn, the other way (x86 to PowerPC) to work reasonably well. Then why have windows emulators for the mac sucked for the last decade???