I've been really satisfied with the Kurgo. It does well both as a car harness and walking harness. It's sturdy, seems comfortable, and has a handle that's great if you need to hold them or quickly pull them out of something for safety reasons. It also has the option of back clip or front clip leash walking. What's really convenient too is the car attachment. Instead of putting a car harness on and off when we get to our destinations, all I have to do is clip the caribeaner on and off. The only drawback I've really seen is that it takes a while to get used to how the sideclips work. But once you understand it, it's great. In terms of total car safety, I wouldn't really say this is a "Crash tested" sort of harness, but I think most of them are treated more like tethers rather than safety devices.
This is the one I have but they do make similar designs that are "enhanced strength" too.
I have an IDC harness and I was thinking about buying one of these car seat belts for the harness. I would think it would be nice, since I have had the harness for awhile now, and never had a problem with it.