I have a pair of 109R1212H1011's. As was said, too much motor noise when undervolted. I prefer my 9G1212H101's.
But my favorite fan is the 9S1212P4M011. I was able to get 8 of them. They're perfect for CPU heatsinks. A fine match for GT AP-15's, but PWM. I have some 9S1212L401's, and they're great, but not PWM. But they did cool my 4GHz overclocked i7 860 to 55.5c at only 19.5dBA, no small feat -- check out the link in my sig. I put one in my daughter's quiet rig. You never hear it.
For GT's, best is AP-14 for all-around use, but I use an AP-12 (800rpm) on the front of my case. I use my spare AP-14's around my case at 5v to get 750 rpm.
Best performing fan: TY-140. Excellent, probably my best, but I'm not in love with it and I don't know why.
And for fun, the 14cm Shark at 5v or 7v pushes a mild amount of air, quietly, with pretty blue LED's.
You want to convert your highspeed GT's to PWM? Look
here.