Some of the best criticism I've recieved has come from people just trying to be pricks.
Example: Someone comes into the MP3 Clinic, and only has 7 posts...and they're all in his thread about his song. 2 of those posts are him "bumping" the thread or posting something like "Come on! Anyone????" I then post in their thread, reminding them that this is a community. He needs to stop complaining about the lack of responses and start giving feedback on other people's songs, too, if he expects the same in return.
So, this obviously pisses them off and they go right to my site and listen to me shit, which wasn't my intent when I posted. They then come back and post that they listened to my shit and rip into everything from my guitar sound to my vocals, etc....
My initial reaction is to blow if off since I know they're just trying to get under my skin since I pissed them off. The thing is, the 2 or 3 times this has happened, I ended up going back and listening to my own stuff and realized that they were right about a few of the specific things they criticized. So, it got me working on it and, in the end, I improved my sound and my ability to listen objectively to my own stuff. It's helped tremendously. That's one thing that's hard to get from "fellow-musician-internet-friends". Sometimes it takes a situation like that get someone's real opinion
Having said that, I'll usually go listen to someone's music before I take them too seriously. Within 20 seconds of listening, I'll know whether I'll ever give a shit about what they say about my music. Even though I know my music is not Greg's style, he has one of the best critical ears here and always nails things that most people wouldn't catch. There are others that I have nothing against on a personal level, but their opinion means shit to me because I've heard their music.
(Besides my 9/11 conspiracy rants in the Cave a couple of years back, this might be my longest post ever.)
