Works surprisingly good, I get over a hundred frames per second one some terrains. 😀 and that's on a 2.4 GHz 15" MacBook Pro
Rigs of Rods Version I have is the newest one available now: 0.38.67
I started with a minimum setup and added content manually (when I included the huge content pack right from the start, I couldn't get it to run on an earlier version).
For starters: Don't unzip the content packs. They go into the packs folder as a .zip
P.S.: Needs Direct X 9, I use the one available in the crossover installing software menu, works.
When setting up the game, you can choose to use Direct X or Open GL. Can't figure out which works better, they work both and there's lots of graphic settings to fiddle around with. If you get bad framerate you can turn off a few things like vegetation and improve it that way.