I've used that set up before, going back to it in a couple of weeks (in fact, using the Traveller because we're shooting in an apartment the size of the heel of my boot). I like it, it's cheap to rent, easy to transport, pretty solid.
We used the four wheel version with the Flex-Trak. The Flex-Trak was a pain because it shifts every time you go over it. I was ACing that gig so focus marks became a little useless (shooting wide open on a 35mm prime lens adapter - very narrow, but we swung it). Flex-Trak might have been more workable on a flat floor (we were shooting in a condemned police precinct in Brooklyn) but you gotta be working in a hospital or studio. Throw some track or PVC piping in the truck....
The CRITICAL THING TO REMEMBER when using the Flex-Trak/Spider combo is to keep the adjustable wheels OUTSIDE the edge of the dolly platform, and on the OPPOSITE side to the weight of the operator (assuming they'll be riding). When we were trying to figure out how to use the thing the wheels tucked under the platform underneath the DP and I nearly killed him. He didn't trust me for a few days after that, which is understandable - but I redeemed myself by figuring out how to make the thing work safely for the remainder of the shoot.