Does this mean they'll be using V8 too?
A year ago I worked on an app with heavy JS frontend and frontend guys told that they can't make it work properly on Opera.
I'm sort of including dom<>js relations as part of the js problem tho.
On the other hand, DOM APIs are rather less interoperable across browsers for both historic (lack of clear standards until recently) and technical (larger surface area, faster pace of addition, lack of proper testsuites) reasons. Therefore they are by far the more likely place to run into an interoperability issue.
Opera really follows the standards, and if I found any bugs they were often an indication of a fragile part of the code that needed fixing (Opera was very useful for finding dodgy areas of my code).
Of course it depends upon the features one is trying to use, not just the quality of the code :-)