Well, I'm not into black/death/doom for the most part, but I LOVE growls if they're done right (completely subjective), fit the music, and interplay with clean vocals. Opeth was the first band that intrigued me, but I still had to warm up to them, so I put Opeth on the back burner for a bit.
I'd have to say Ensiferum (folk metal) was a turning point for me...hearing their album Iron, something about Jari Maenpaa's growls really resonated with me, along with everything else about that album. From there discovered other melodic/symphonic/prog/gothic bands that employ combination vox like After Forever, Epica, Orphaned Land, Penumbra, Adagio, Sirenia, Wintersun, Therion, Kiuas, Elvenking and so on. Of course I revisited Opeth, and became very fond of their recent albums (Still Life and forward).
So now I find that when a largely clean-vocal band throws in a small to moderate amount of growls, I'm usually all over it...and it seems to happen more often now as well...perhaps some genres/bands are breaking down old barriers.