I certainly base my opinions on the music. I love System of a Down, I like Metallica - Load, I like some Offspring....Why anyone would dislike a band just for being mainstream, I just don't know.
Now, my definition of 'selling out' is 'watering down your music so that it appeals to those who didn't have the mental capacity to appreciate it before'. If a band does that, and I don't like the music anymore, I won't waste my time with them anymore. I also won't make about 100 threads about them moaning and whining (cough cough St Anger threads cough).
So basically, stop moaning. If a band you loved suddenly went mainstream but their music was just as good, would you enjoy listening any less? If so, then you deserve to be upset, and whining about it on here isn't going to help. I say good luck to Opeth, if they get as big as Linkin Park and keep their music at a high standard, that's great.