I've been through many CPLs lately. Started with 58mm and 72mm Fujiyama i got cheap in hong kong.
Then trawling ebay one week I came across a 67mm B+W Kaesemann and 77mm Hoya HRT, and then a week ago I got a 72mm Heliopan ES72.
The original intention was to put the Hoya HRT on my 70-300L, get better bird colours (like for Rainbow Lorikeets) and not lose too much shutter speed. But the Kaesemann wins hands-down for the 70-300L.
I did some direct comparison-shots on the weekend, I haven't gotten to posting them or writing a review, but in short, the Hoya HRT messes with the colours way too much. Surely I could just PP it back to normal, but the white balance is off and skies look brown. The others look all about the same.
And whoever said flare, that's another main problem. The Kaesemann is the only Multi-coated one I've got (i don't think the heliopan is mc, i'll check when i get home). Here
's my original test before I got the heliopan (or a good tripod), i'll be posting the next test results at some point...