Webkit is basically the defacto rendering engine for the mobile web right now, being used by virtually all mobile browsers on all mobile operating systems with the exception of WP8, which is only like 2% of the market anyway, and a good portion of the desktop space, too, thanks to Chrome for the most part, and to Safari on Macs. Webkit is also open source, which is why so many companies have adopted it.
This Opera Beta app also includes the Off-Road feature, formerly known as Opera Turbo, which is basically what Opera Mini does, but integrated into the full browser, and you can switch this mode on or off. With Off-Road you should be able to browse at least 5x more with the same amount of data, although keep in mind that all this data needs to pass through Opera’s servers to be compressed, so you might not want to login to your Gmail or other important accounts while in this mode. Use the full browser mode (Off-road off) when doing that.
The new Opera also seems dedicated to serve the low-end and old devices market, considering its menu is still very much Gingerbread-like, instead of being contextual like in ICS and Jelly Bean, and it also comes with support for ARMv6 chips, which are found only in the lowest end smartphones. Chrome and Firefox don’t support that architecture right now, and probably never will (although Firefox has been working on an ARMv6 version, but they might never officially release it, considering even low-end devices will start getting ARMv7 chips like Cortex A7 or even Cortex A5 this year).
From the reports so far, it seems the new Opera app is very fast, and it deserves at least a try. Although, keep in mind it’s still beta for now, as the name suggests, so don’t be too harsh on the reviews.