Fortunately I did not buy it yet, and I think I'm not gonna buy it ever... (I just used free and trial versions).
And I think you understand, that even PRO version of GS doesn't really work out 800 bucks...
And it's really can't compete with such game engines as UDK and Unity3D (which you heard is really cool).
Most of GS users aren't professionals (as me), so programming with GS is just a small hobby for them.
But to get normal version (with shaders and so on) you need to pay about 200 bucks...
Why do you need to pay 200 bucks, if you can get free version of UDK with great PhysX and shaders?!
The only thing that keeps me with GS is LITE-C... It's really great and easy language, but the way GS uses it, is sucks...