How good are Tripod Dollies?

They're ok. I owned a couple of sets of the manfrotto dollies and they worked well only on flat floors. I used to do a lot of live work and we would do stuff in school gymnasiums, obviously they rolled smooth in there. if you need smooth track shots, your best bet is to get a dolly on a track or find a different method of stabilization (stabilizer, crane/jib, etc). What are you trying to accomplish?
 
I would mainly use it to shoot tracking shots outdoors, and I was wondering if cardboard on the floor could perhaps smooth out their movement across street or sidewalk pavement.
 
Cardboard is fine until it hits the seams. Most concrete and asphalt will be rough (unless its smooth like in a garage). You're not going to get a smooth shot with it unless you're in ideal conditions.
 
It's not going to be good. For one thing you'll have to doing some heavy shimming to make sure that the surface doesn't bounce when you roll the dolly across it. And cardboard won't cut it, too light. You'll need something like a large and fairly thick wooden board to give it a stable surface. Those dollies have really small wheels which are not great for getting rid of vibrations, and you can't even lock the wheels rotation so it'll be very difficult to get a straight shot without the dolly veering all over the place.

If you have DIY skill I would get one of these dolly, remove the stock wheels and replace them with skateboard wheels, so you end up with something like this

http://www.flexdolly.com/flexfullthree.jpg

Get a bunch of PVC pipes and there's your dolly.
 
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