Current Opera’s rendering engine Presto still has trouble with some websites, and then, WebKit is a really nice rendering engine, too bad that so far all of the WebKit browsers suck balls.
Safari is no longer developed for Windows, Chrome has become really slow with almost nothing new happening there. 3rd party WebKit browsers as just a pain to use as they are extremely unstable and lack so much important features.
Opera is about leaving a small memory footprint, so I hope they will keep it that way even after implementing the new engine.