The new version of Opera using the webkit rendering engine instead of the Presto one uses a misleading numbering system. In order to make the new version available as soon as possible they have simply started from the open source chromium version (the same open source basis for Google Chrome) and changed its appearance to use Opera branding. They then started adding Opera specific features into it. They then made it available for people to use while they moved on to working on the next version that adds in more of the features you'd expect Opera to have.
The only thing wrong with this approach is the way that they have named these versions with higher version numbers than the latest Presto based version of Opera which has given some people the impression that the latest of these versions is in fact the latest version of Opera whereas in fact most of the features that make it Opera are still missing from that version.
Only once they release a webkit based version of Opera that incorporates all of the features present in the latest Presto version of Opera will the webkit/Chromium based version actually be able to be considered to be a replacement for the presto version. The reason most Opera users actually use Opera is because of some of the features that Opera provides that the other browsers do not (or didn't until they copied the feautre from Opera). While the webkit based Opera has fewer features than the other browsers it doesn't really deserve the name Opera.
I fully understand the reason why Opera is actually releasing versions of the new webkit Opera for people to use. If they didn't there would be no new version of Opera until they finish the rewrite. At least by releasing these new versions people will be able to see the progress that they are making. It is also possible that they may have some people decide to actually upgrade to that version prior to all the features being added in as not everyone makes use of every feature of their browser. Their approach means that once they have added all the features that a given person expects Opera to have then that person can upgrade to the new version and not lose any of the functionality they expect from their browser.
The only problem with the approach is the numbering system that they have used that implies that versions 15, 16, 17 etc are upgrades to version 12. The automatic update option within the browser itself recognises that these higher numbered versions are not yet ready to be considered to be updates to version 12. While the program itself knows which is the latest version though, many people do not and I have seen forum posts where someone claims that because version 16 of Opera does something differently than version 12 that Opera has abandoned the approach that version 12 uses and that any issues they have with that approach can therefore be ignored. In fact the only reason I can see for that difference is that the new version of Opera just hasn't implemented that particular feature yet.
There are a few things that I would expect to see in the new version of Opera before I would consider switching to using it.
These are the most obvious things that occurred to me when I first looked at the new webkit based version of Opera so there are probably other things that I use in the current version of Opera that the webkit version does not yet have. When the new version actually implements all of the things that I know that I currently use in the current version I'll take another look.
This article written by Stephen Chapman, Felgall Pty Ltd.