First of all, I'm a BMXer, not a road biker, which means regardless of what kind of bike you're buying this for you need to listen to my version.
I finally took this thing out and tried it at Challenger 7 Memorial park, I took it on the back trails. This isn't a well known biking spot, but anyone who's been on the back trails there knows it's damp soil and tree roots. This thing gripped my EVO without a problem even while I was doing wheelies and bouncing over root systems at high speeds. I did have a little trouble with neck swivel, so I just tightened it up a bit more, no problems. I doubt many people considering this item will submit to 1/4 of the bouncing and stresses I do on a normal ride - most BMXers wouldn't even consider using something like this. Only a real downhill biker would subject this to more abuse than I have, and I'm interested to hear from a downhiller.
On to other details. My bike is a 2008 Diamondback Lucky 24 (as of right now the PDF back catalog is still up on their website if you want to look - the BMX catalog was separate that year, pg 11). The cross member on the factory handlebars, which I'm still using, are thicker than you would expect from most handlebars. I ride the Galveston seawall at night a lot and the standard headlight wont attach to it (I figured out another way) but this thing fits perfectly without slipping. The way it's designed I have no doubts I could put it on a thicker or thinner crossbar without a problem. To make things better, it's tool less. If I want to take the thing off I don't have to get out my screwdriver or wrenches to do it.
If you have a need to mount your phone, GPS, or music player on your handlebars, this is the way to go.
-------------UPDATE 09/22/2011--------------
The above review holds true, but I have to lower it a star now.
Previously I had an Otterbox on my phone. My factory battery wasn't holding up anymore so I got a new one, one of those long life batteries that requires a different back on the phone. The Otterbox had to go, and wasn't holding up anyway due to the kickstand causing a structural weakness.
Big battery now in, Otterbox gone, riding my Comfort bike, not one of my three BMX's this thing dropped by phone and shattered the screen. Has to lose a star, though I do stand by it as a good choice if you have the secondary protection I used to. EVO 4G bites the dirt.