I just ran a very strange extremely un-scientific test.  I ran the same image for 2 minutes, first with CPU only then with GPU acceleration.  

CPU only had .86 S/p at 21 kS/s.  GPU accel got 3.8 S/p at 102 kS/s.  

**But **the CPU only image looked better and was clearer.  Why did that happen? I don't normally use GPU accel because I like to play games while a render is running and I never saw much of a difference in speed the few times I tried it, but I can't understand why the image looks so much worse if it's running faster by the numbers.