I just picked up the Foamwerks cutter that works with their channel rail thing...works great. I plan on doing a few more foam core projects, and being able to make cuts that are perfectly straight (not 2 or 3 degrees off perpendicular) is awesome. No matter how hard I tried when using a craft knife, the cut would be a little off. I guess the pressure when sliding it along the metal ruler tilted the blade a few degrees, so that the cut was a little beveled.
Before I got the Foamwerk cutting system, I tried using just their rail knife and their non-mounted ruler/rail. I still had a hard time keeping my 90 degree angles. The board fixed that. I can see making a couple of jigs to help make smaller pieces. Overall a great tool.