I bought the vent mount model about 2 months ago and it worked without a problem with my Eris for about 2 weeks. Then it began to fall off constantly while driving. I found I had to press the phone into it very firmly with two hands to make sure it sticks or it will fall off after about 5 minutes.
Just got my Incredible today and it has the same problem, I have to really force it onto the sticky pad or it will fall off right away. Also I have to make sure it is far enough down the back so the camera lens isn't anywhere near it or it falls off right away. Just bought a textured case and belt clip combo tonight but haven't tried it out with that yet. But am assuming I will have to press it in even harder to get it to stick.
I only paid $14 for it at Wal-Mart so its not a big loss and I haven't found any thing else I like that works better. Most mounts are just too big and bulky for my taste or too expensive.