I'm not sure how I first heard about Opeth, but I think it was around 2004-2005. I was listening to stuff like In Flames at the time and hadn't really been exposed to growls yet. Also, the music i listened to was mostly fast/heavy and didn't have the emotion that a band like Opeth puts into it.
I somehow got my hands on Damnation and found it really intriguing. I had to be in a certain mood in order to appreciate it, but the seed was planted. I had tried listening to their other stuff but was immediately turned off by the growls and never really gave it a chance.
A couple years down the line I started branching out into wider varieties of music. I had just been through a pretty emotional breakup, so I was listening to stuff like Katatonia a lot. Through their website I saw that there was a new Opeth album out.
Out of curiosity I decided to download it (I know, piracy=bad, but i own every opeth album now so whatever) and the first track I put on was Harlequin Forest (Saw the song length and just had to see what the deal was). I was immediately hooked, and while the growls still didn't perfectly click with me then, it didn't take long for them to grow on me.
Opeth quickly became my favorite band and has been ever since. I listen to all their material (mostly post-Orchid/Morningrise), and I never get sick of the stuff.
Sorry for the lil opeth life story there, but I guess it just goes to show that for someone to get into a band it takes more than just a specific song. A lot of people that listen to Opeth listen to them because of the emotions they incite in the listener. If you've gone through a hardship it makes it a lot easier to take in something like that. After trying to show Opeth to many many people I finally realized that you can't necessarily get everyone to appreciate them the way you do... or maybe just not yet.