We did two 17 hour moves from MA to TN and then the reverse, both moves with cats. We let the cats walk around, but we had very well behaved cats and there were enough of us to keep them away from the driver. We just so happened to have cars with "way backs" so we put a litter box in the back and the cat could walk around. On the move down, Mandy was so cute sitting on the dashboard!!
I did a 4 hour drive with Chloe. She loves riding in the car and was no problem in the carrier, but I could see her being a problem with the driver's feet and don't know if I would want her loose. I, too, think it depends on the cat.