I can't remember when exactly I first got into Opeth. I know that it was either late '04 or early '05.
In any case, my first experience with them was Blackwater Park. I didn't understand all the buzz about it at first. I listened to it a few times over a couple of days. I liked "Bleak," but that was it.
I shelved the album for a week. Then I couldn't stop thinking about all of the raving about Opeth that led me to buy the album in the first place, and I examined it again. "How can so many metal fans be wrong?" That's what I was thinking.
Well, I fell in love. I've always been that way; if a widely acclaimed album or movie or novel doesn't sit right with me, I try to figure out why it worked for so many other people. Many times, this leads to me "getting" something. I'm not a sheep, though - if I can't understand the appeal of something in a reasonable amount of time, I'll stop trying and just realize it isn't for me. I like to keep an open mind, but you can't get a person to like anything and everything.
Anyway, I immediately bought Deliverance and Damnation. A few months later, I picked up Ghost Reveries right when it came out. I slowly picked up the rest of their back catalogue. I thought they were my favorite band ever.
My favorite Opeth albums, in order:
1) Ghost Reveries
2) Still Life
3) My Arms, Your Hearse
I will admit, though, that there have been long periods where I had grown bored with Opeth. By the middle of 2006, I'd picked up all of their albums, and for most of 2007 I thought I'd gotten over them. Maybe they weren't my #1 band of all time after all. Maybe they were just a fling. Even my favorite songs didn't seem to be doing anything for me anymore.
Then, this last December, I threw in Ghost Reveries on a whim. It was glorious, even better than before. Same with Still Life, and, heck, even some of my lesser favorites like Morningrise.
I realized what caused my problem: I loved Opeth so much, that I practically listened to them almost exclusively through 2005 and 2006. I wore out those albums, hardcore.
Amazingly enough, though, it only took a year break from Opeth to love their stuff more than ever. Yeah, I can say for sure now that they're my #1 band. I'm cautious about listening to them too often now, and it pays off.
To the OP:
Yeah, I noticed that a lot of people do seem to fall head-over-heels in love with Opeth, then forget about them a year later. I think those people just overplayed the music, like what happened to me. It's kinda hard not to listen to one of their albums over and over and over again when you first hear them, and that leads to massive burnout.