Slightly late to this party, but my two cents, for what they are worth...
I will continue to use Opera 12 until such time as there is another browser that has a comprehensive feature set that covers most of the features that I use – whether that's Opera or another browser – or until such time as it is so far behind the curve that it is unusable on too many websites.
The whole reason I have been using Opera for more than 12 years is because of those features. Without them, there is no reason whatsoever to choose Opera over any other browser. Its features were what made it the best browser, and it's a shame that they never had the marketing budget or strategy to make it more popular. I don't understand why some people say it was "too complicated" – for people who just wanted a basic browser, it did everything that other browsers did just as easily, but the difference was that there was more there if you went looking for it.
It's now been nearly 18 months since the public beta of Opera 15 was launched, and we are now up to v24, but each of those 9 new versions seems to have done little more than add a single feature at a time. At the rate, we'll be up to Opera 50 at least before the feature list is restored to a level that will satisfy those of us who got the most out of Opera before. But is it going to happen? I would be a lot more accepting of the changes, and a lot more patient, if I could see some definitive statement from Opera ASA to the effect that they are working to reinstate those features from Opera 12, and to give a rough timescale of when each one might make it back into the browser. At the moment, it feels like those of us who were the biggest advocates and evangelists for Opera before have just been discarded with no consideration given to the fact that there are no alternatives available that do what we want, and we have to be content with whatever crumbs are deigned to be thrown our way.
I get that Opera ASA is a business, I understand that. But a business is nothing without its customers. And if you alienate your entire existing userbase by removing your USP, and instead enter an already crowded marketplace with competitors who are bigger and louder than you are ... well, I'm not so sure that's a great business decision.