A lot of people don't like Opera because it has some compatibility problems with sites sometimes..it follows W3C standard very strictly.
However I've gotten so used to it's interface and speed that on multiple occasions I have tried switching browsers and have never been able to stick with any other browser. I've tried to force myself to use Firefox and Chrome and I always find some annoying quirks that have me running back to Opera. It's all about personal preference, IMO.
They should standardize browsers more so that anybody has the choice to use any browser and they don't have to worry about the site being compatible or not. This problem is much less than what it was 5 years ago when IE was the most popular browser, but it still happens from time to time.