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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. The effect on security just depends on how it is done under the hood.
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.