From trying it on Windows, you're not missing much. It's mostly Chrome with a different skin as most of the features users liked on Opera < 12 are gone or have lost functionality. I still use Opera 12 for that reason.

Some of that functionality may return in the future, but they have said that things like bookmarks will never be like they were exactly on Opera < 12. If they had lived up to making it Opera 12, but with a Chromium Engine, it wouldn't be a problem, but as many long term users were afraid, someone at Opera is using it as an excuse to radically change what Opera is.