A little belated response, but for a smallish robot, like shown in reply #7, I would imagine that motor/wheel arrangement will be fairly reliable. After all, Lynxmotion has been selling those things for years.
OTOH, if you're talking about 6" wheels and a 2' x 3' or so base, that's pretty heavy-duty, and you really need to go with strong motors and probably wheels mounted on shafts riding on bearings, and connected to the motors via pulleys or chains.
http://damencnc.com/en/components/mechanical-parts/ballbearings/279
Skid steering works, but as mentioned, the carriage needs to be extremely robust for something that size and weight, and they certainly work best on surfaces where the wheels can, in fact, "skid" easily (eg, not carpets).
It's very common for people to use casters as a 3rd wheel [tripod arrangement] for differential-steered bots, but you'll notice this thing is "tiny" compared to what's being talked about,
A large, heavy frame will tip over if not properly balanced.