I used to use Opera a few years ago, but its Javascript performance was iffy. By the time they fixed it, I was already hooked on Firefox, and if I recall correctly, Dragonfly wasn't as mature. After switching to Mac, I tried Opera again, but it -still- doesn't support inertial scrolling, so I'm currently using Chrome.
JavaScript performance was iffy? Opera always had the best JS performance by far, except for that year or two when everyone else started using JIT.
That was just an anecdotal comment - it's been too long for me to remember any concrete examples. I haven't had any problems lately though.