Opera 10.51: Unimpressive
Okay, the newest Opera is definitely not an improvement over the previous version. I've been using it for a couple of weeks and it suffers from weird bugs, including Facebook randomly signing out at the strangest times and pages not loading due to waiting for DNS connections.
Also, it's decided to ignore English spelling in favour of American spelling and it's added a couple of GUI changes that were not particularly helpful (hiding the menu bar, for instance) which had to be changed back to the way they should be.
This is the most disappointing Opera update that I've experienced, the Opera team seem to have spent far too much effort on superficial features without making sure that it still works.
On the bright side, it has more support for HTML5, which isn't much of a bright side for the average user. And the tab controls look a little slicker, which is nice, but still superficial.