I used sbr12 on my machine, on all axis, I have had no real issues yet other than the adjustment set screws on the bearing blocks vibrating loose, then the blocks lose their rigidity, but I think that goes for any supported round rail. They are very rigid and loading was more than sufficient for my machine, 20-30 lb steel gantry. I haven't tried aluminum yet and probably have less than a hundred hours on it so far but I am very pleased. My machine is 28" x 18" x 8" or 650x440x200.
Here is link to my build thread if you want to see it.
http://www.cnczone.com/forums/cnc-wo...16384-cnc.html