I tried Opera, long ago, but at the time it was "pay" or "have some annoying ads!" and my browsing needs were minor - I think we're talking early IE4 days, and I didn't have a computer of my own yet. The tabs were awesome, then, and I could see the future, but I felt no need to pay for it.
Then I was on Linux for a long time, which was the first OS to get browsers that copied Opera's tabs successfully, and never had a very good actual Opera version while I was using it. I've been using Firefox since it was called Phoenix (and maybe Firebird at some point? I've forgotten the deets) and before that I used a slate of linux-only browsers that used the Mozilla engine (Galleon was one whose name I recall). Firefox et al were an obvious choice on Linux because of the whole Open Source Unites! thing.
Then I switched to a Mac and used Firefox immediately because my other choice was IE (Safari wasn't out yet). Used it until Safari came out, and switched for a while. Then I discovered true love with Camino, and have had no need to look at other browsers since. I did download Opera for Mac once, recently, but wasn't too impressed. I might try it again, but for me, Camino is home now. I doubt I'll switch without truly compelling feature differences, which Opera isn't offering me anymore.
That story was way too long. Sorry. I feel geeky now. Especially since I just reread HP5 this weekend, in lieu of getting shit done.