Opera warns those who download the software that this is an alpha version and hiccups (and even blue screens) may occur. I found that those warnings are, in fact, reasonable and not exaggerated. Opera 12.00 alpha is a very early preview that has plenty of issues and should not be used for everyday tasks.
What is new in this browser is acceleration for WebGL content as well as well as full hardware acceleration that goes beyond the previous software-accelerated backend Vega. When Opera says "full" hardware acceleration, it means "full", as there is not just acceleration for features such as Canvas, but also renders the entire user interface of the browser on the GPU. There is only a hardware accelerated OpenGL backend available, however.
It is difficult to get an idea how well hardware acceleration will work in Opera 12. I noticed that benchmarks such as Webviz do not run at all in Opera 12 and the IE Fishtank WebGL port from Mozilla shows errors in Opera 12.00 alpha. However, the Asteroids Canvas 2D benchmark delivered a score of 2165 on my Phenom II X6 1055T system, which was up from 1580 of the previous Opera Next release without hardware acceleration. In comparison, Chrome 14 scored 2575, Firefox 7 1227 and IE 9.0.3 2864. In Microsoft's Mr. Potato Gun test, 12 alpha scored 15,399 points versus 1141 of the non-accelerated version. However, Chrome scored 34,876, Firefox 7 34,988 and IE 9.0.3 52,344.
It is too early to draw any conclusions how fast Opera 12 will be in those tests, once the browser is more stable. So far, it is just a download for die-hard Opera fans. Originally, the hardware acceleration feature was promised to be released with Opera 11, which was announced in December of last year.