Except this is an engine that can be fixed by anyone when it's broken.
I'd agree that the software monoculture aspect of it is less attractive - webkit is going to have to be very secure!
I've been using Opera for years. (Since v3, and I paid for several versions.) I'm typing this in Opera right now. It's a great browser. But there are some websites that it still doesn't quite work with.
Nothing major - and often worked around by simply using Site Preferences to spoof the user-agent. But it's not perfect.
Neither is Chrome, nor is Firefox. They all have either rendering behaviour or an interface quirk which bugs me. Opera just bugs me the least.
This at least means that Opera will be feeding back code to WebKit. Perhaps WebKit will get an option to handle zooming in Opera's much better way. Perhaps some of the excellent work on reflowing that Opera has done over the years will slip into WebKit.
Overall, this looks like a good thing for all concerned...