I've tried a few different purchased chocks. For my use, I've settled on the black rubber ones in the post just above this one. They are the ones I trust when I'm chocking my rig at a launch ramp (and prefer to only launch the boat not the van too!). BUT they have some distinct downsides. One is they are quite heavy (as compared to others) and two is that they permanently stink of strong chemical/rubber smell (or at least two years in outdoor/ventilated space has not taken away the smell). But they are really grippy and I trust them.
I've also used the yellow hard plastic type. They are very light, and don't smell, but can sometimes slip and also will break if you accidentally run over them. I do keep one in the driver stepwell of my van just for quick stops where I might want it (and there's no way I'd keep one of the black rubber ones inside).
The third type I have tried are the orange urethane ones. They seemed to be very high quality and don't smell, so I had high hopes for them -- but I found them to be too slippery for my use (darnit). They were this type:
http://www.amazon.com/Roadblock-UC1...1464041934&sr=8-3&keywords=orange+wheel+chock
I kind of knew there might be a problem when one of the reviewers said he loved how they would slide. He used them to slowly "slip" something down a ramp. But they probably work great for certain situations.