I had good results with solid rotors on my kdx. Brakes seemed to last longer, and also faded much less after encountering mud, puddles, etc. If\when I replace my stock rotors, they will most likely be solid rotors, unless the cost is prohibitive.
Yeah I would think they wouldent be able to disapate(sp?) The heat fast enough and then would cause fade? At least in a race where your breaking alot, I would think just woods riding they would be fine thoe.
I never experienced any problems due to heat, etc., with the solid rotors, either in the trees or on the track. To be honest, I was pretty bad about dragging the rear brake often on the kdx. Although the brakes would squeal occassionaly, I didnt experience any fade, etc. No problems with solid rotors, and only good things to say about them.
I put a solid rear rotor on my '99 KTM 300 EXC and brake pad life increased dramatically. With the stock vented rotor, I was wearing out a set of metallic pads in less than 50 miles in mud. With the solid rotor and organic pads, I get approx. 500 miles per set, with no noiceable difference in feel or fade. If you ride in the mud, a solid rotor is an extremely good investment. Saves big $$ on pads.