Opera 12 alpha builds have been superfast in my own experience. And the arrival of beta is good news.
I think that this is going to be the best Opera release in a very long time. It is fast as crazy.
I personally would not be able to switch to it from my old installation because of what I use it for but I recommend anyone reading this post to give Opera 12 beta a try.
Opera has done the impossible. Web pages load so fast on this build that it seems to have outdone the competition from both Firefox and Chrome.
Quick startup with a smarter tab-loading sequence
Faster page loading, even for secure webpages
64-bit support for Windows and Mac
Smooth and stable browsing, thanks to support for out-of-process plug-ins
Redesigned security badges
Camera and extensions APIs
Right-to-left scripts support
Experimental hardware acceleration WebGL support
New web standards support
Discontinuing Opera Unite, Opera Widgets and Voice