I have a B+W MRC CPL 72 mm thread for my EFS 15-85. I am sometimes not completely pleased with color temperature with it on, but I am very pleased with the effect and sharpness. I have a B+W Kaesemann MRC CPL 77 mm thread for the 10-22 and 100-400. I actually don't recall any similar concerns on color temp with the Kaesemann, but I use it significantly less and have never really looked.

So, I am pretty happy with the two CPLs I have and would recommend them, I have been tempted by the Singh Ray "LB" polarizers (get polarization while sacrificing less light--both a pro and a con depending on how you want to use the filter) and the Singh Ray "Blue-n-Gold" polarizer. I believe Jonathan used the Blue-n-Gold for his recent artic trip to great effect in a couple of shots.