Opera man here. Began trying it intermittently a few years ago, but always fell back to Firefox and it's extensions. But I started to fall in love with Opera after 9.5. Then Opera 10 came along under the banner of being 'the fastest browser on earth' and any lingering doubts disappeared. Great interface, email client, found a few useful things to do with Opera Unite. I've never looked back.
Firefox 4 looks like it took blatant visual cues from Opera 10+. Have used Chrome recently and Opera is definitely at least it's equal. Not sure extensions are yet up to par with those of other browsers and ad blocking on Opera has always been painful compared to other browsers but you have to take the rough with the smooth >.< Only reason I use Firefox now is to view those websites that still can't get their heads round Opera.
Opera is a good browser, but falls behind in a few speed related tasks. Mostly that is to do with javascript execution, so its not noticeable on most web pages, but if you use javascript heavy web apps, you would notice the the difference. That said the speed between the browsers is really quite close now, with the exception of the canvas element for HTML 5. Firefox and Opera fall behind IE 9 and Chrome on that, though I've not tested Firefox 4 in that regard.
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