I've been using one of these for a little over a year now, love it. I don't transport a car with it, but it easily carries parts, spares, fluids, tires, etc to support 2-3 classes when I race. I use a separate hard case, similar to this one
, to hold my tools.
I use the lower bin to hold all the spares and tires for the cars I'm racing that day, the removable tray and center section hold my "universal" stuff [shock/diff oils, soldering equipment, tapes, car stands, etc]. So when I'm switching between onroad and offroad, I just have to unload and reload one section.
If I raced just one class, I'm sure I could condense down what I carry to fit into just that one box though.