Opeth is an aquired taste, at least they were for me. I remember buying the album Still Life roughly 3 years ago and initially I wondered what all the fuss was about. I'm so glad I force fed myself that album, because the phrase "it'll grow on you" doesn't even begin to describe how different the music sounded to me after quite a few listens.
Soon after I started buying the rest of their albums in this order:
My Arms Your Hearse
Morningrise - my current favorite
Damnation is the mellow album with no growling, just singing. I like it, but I love the contrast of the two vocal styles together.
Ghost Reveries has such sweet production. Even though it's a metal album, I would have a hard time finding another album to match it in sound quality.
Morningrise doesn't have near the production, but it's so beautiful once the melodies click (this will take numerous listens, this is a very progressive album).
My absolute favorite song is on Deliverance, and it may be their most accessible. It's called "A Fair Judgement" and it has the most beautiful guitar solo I've ever heard. I usually don't think about the guitar solo first when I think of a song, but I do with this one.
I highly recommend trying to get used to the growling vocals if you can, because if you do start to like them you will have 7 amazing albums chalked full of GREAT music