I'm not a fan of death metal vocals usually, but I got used to Mikael's take on them. Knowing how good his clean voice is helps me take it more seriously I think. And like everything else Opeth does it was used intelligently, as an arranging tool, a way to build tension in compositions. I started with Damnation and had to gradually acclimate myself to the harsh vocal tracks.
The musical arrangements in those songs is what got me to stick with them. There are 3 heavy Opeth tracks that really tipped me over the edge into appreciating the arranging power of alternating between clean and harsh vocals. The title song Deliverance from that album, Ghost Of Perdition and Baying Of The Hounds from Ghost Reveries. It's also worth pointing out that all 3 of those songs are AMAZING! After those tracks "clicked", I no longer minded the harsh vocal songs. But Opeth is really the only group I listen to that uses that style.