Yeah, I've checked them out. Definitely an option, but (for instance) a 2"x5"x10" block is $30. That's twice the cost of polypropylene. Of course, the machinability adds value, but I'm not sure it adds that much.
Of course, I suppose you can melt down and reuse the "scraps," which is definitely a huge plus. I've noticed that when cutting polypropylene sheets, I probably am only able to use about half the sheet, since I have to have somewhere to clamp and the various parts I cut don't always fit together well.
Actually, for that reason alone it's tempting. You've done resin casting in machinable wax right? Are you able to cast and remove parts without mold release? I imagine (being wax) that it's pretty non-stick.