I've never heard an Opeth song. What's a good one to start me off?

Reading along with the lyrics while you listen to a growled song is a good way to get used to the lyrics. At first, it's very hard to understand what he's saying, but now growls by most bands are a lot more comprehensible to me, and I just love them (esp. Dan Swanö, and Mike). So, go to darklyrics.com or some site with the lyrics and just read along. That was what bridged the gap for me between tolerating the growls and loving them.
 
The Drapery Falls and Moonlapse Vertigo are good songs to get into growling, you should try them. They have both soft/heavy parts and clean/growled vocals.