On the one hand, it's fucking GAY that Opeth is becoming mainstream, and I totally admit that's my purist/snob/immature fan side.
On the other hand, good on them for getting to a point where they probably can stop eating beans eggs and toast for breakfast/lunch/dinner, and support themselves and their families comfortably.
Another point is that this is good for music as a whole: the real metalheads here know (I'm excluding the typical Opeth fanboys from this because you don't count and don't know shit) that Opeth are one of many creative bands in the extreme metal world. That they have been able to break out like this is a good thing for rock music overall.
We have been plagued by shitty rnb pseudo-music for too long now, as well as "pretender dirtier rock music" in the form of hardcore and emo. It is about time that a true musical force emerged in the more extreme side of rock-based music, and their increasing popularity may well educate many around the world in terms of what rock based music is capable of in 2008, given they've grown up not knowing what it's like to have bands like Led Zeppelin, Iron Maiden, Rush, Dio, Rainbow, Journey etc etc leading the way.
So, despite my typical leanings of "oh it's shit because they're becoming popular" I think it's also a good thing for public awareness of what can be achieved with this kind of music, and that just because it doesn't sound all rosy and Celine Dion-ish, doesn't mean it cannot be appreciated as an art form.