I've got one but I haven't had that much time to test.
Went from a X6800 to a QX6700
Also I went from a 7800GT to a 8800GTX at the same time.
So performance increases were pretty much impossible without hours and hours of free time ;P
Anyway, there's pretty much nothing out there to use all four cores, there will be, but not now.
No Issues so far however.
If there is i'll be sure to post here.