I've read alot of similar questions. Really, does it matter if they are at one point highly popular? Is there some inherent flaw in being well known? Led Zeppelin, probably my 2nd favorite band (Opeth being my first, of course), is very well known - yet that does not at all affect the quality of their music - and nor will it affect Opeth's should they reach similar status.
Infact, I think Opeth fucking deserves it by now - mass critical acclaim. I only discovered them last year.. I basically just stumbled upon their name, downloaded some mp3's, and promptly snatched up their whole discography. I was highly impressed with the quality, and diversity, of the music they've produced throughout the years. So much so, I really was flabbergasted as to why I didn't hear of them sooner. I guess if they hadn't gotten as popular as they did, I may have never discovered them.
In short: Just enjoy the fucking music - not the image!