Just say (write) what you think. I've done it lots of times on PhotoSig and nearly every critique I wrote (some of those were NOT flattering, while still remaining constructive) got positive marks ("helpful"). Sometimes I even get mail from the photographer in question telling me that the photo he/she posted was a test to see "if he/she could get away with" some extensive photoshop postprocess work. Personally, I prefer an honest opinion (even when it's not flattering) to the usual "great; excellent... now review my photo's". I'm not in it for the "points" (couldn't care less), but for the "fun" of it. Just my 2 c worth.