In my experience windscreen mounts suck, you can't rely on them to hold your phone and they get in the way of your view of the road anyway.
Dashboard mounts I haven't found to be stable enough, and if you have a dashboard coated out of super duper non-stick like my current one is (for some reason) you've got no chance.
Air vent ones are ok as long as you're brave enough that you're not worried about snapping the vents, which feels like a very real concern since they're only made out of thin plastic 95% of the time.
For me, the best solution was an improvisation on the air vent idea, using a universal holder, a phone-specific desk dock, some sticky velcro, a small chunk of wood, and a cable tie and a couple of wire ties to hold them together. Now I have a beautifully stable dock sticking out of my air vent (I can put the directional slots back in if I ever remove the dock) that automatically charges my phone as well as holds it securely, without me having to fanny around with cables.
https://dl.dropboxusercontent.com/u/2166977/Car/Phone%20dock%20%28small%29.jpg[/IM][/QUOTE]
looks terrible tho..i have a windscreen mount and it gets bashed about something chronic when im offroad..hasnt failed yet.