well, opeth have been a lot more mainstream starting with ghost reveries. i'm not going to say that ghost reveries and watershed are mainstream albums, but they are definitely fairly mainstream metal albums. i've seen 15 year old emo kids wearing ghost reveries shirts, it's disgusting. my point is that artists know when they're going to gain popularity in the business, and they also know what measures they're going to have to take to stay on top. opeth have clearly taken the necessary measures, i'm not going to say that they've sold out, but i will say that i personally feel a diminished integrity on ghost reveries and watershed.
to say "music is music" and "if it's good then it's good" is all true of course, but you also have to take a look at the context of the music and the band's history if you want to make a more meaningful interpretation...