I received my Kensington mount yesterday from buy.com. The mount is very well made. I have the Evo in the Seidio Active case and at first, it didn't fit quite right. The clips didn't really wrap around the edges very well.
However, I noticed that there were sticky rubber pads on the inside of each clip. I removed them both to make more room for the phone + case. You don't need those protective pads anyway if your phone is in a case.
Well, it fits much better now. I wouldn't say it's perfect because you can quickly knock it out if you hit it from behind, but it's pretty snug. I've tested driving with the phone and it's secure. I'm using the vent clips in my 2002 impala...no problems. I don't think they will work with the vents that have deep grills like the previous pictures. You'll just have to use the windshield suction mount.
I love how you can rotate from portrait to landscape easily. I'm constantly switching from both modes and this option is critical.
Overall for $12.99AR from buy.com, you can't beat it!