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I've used opera for years now and I will most probably continue to do so. A major reason is that opera only uses 1 window, others are the speed dial and opera unite.
I'm typing this comment in the new pre-alpha on a virtual xp and it works great. I hope the linux version is just as good. Thanks to Opera Unite I could try my favorite sites very quickly. (when will others follow?)
Dialogs in page is just a very good idea, why stop the whole browsing experience for one page.
I've read about extensions in the comments. I won't say no to them, but the way they are implemented now in other browsers is not the way to go for me. I tried to use them with firefox, but finding good ones for me was a nightmare.