One thing it may be helpful for other Looker users to know before trying to convert to New LookML: after talking with Looker Support, it seems that LookML validation can no longer “see” includes or sets where you have explore definitions outside of a model file (i.e. cases where you want to have a single set of Explore definitions that you re-use across multiple connections or model files.)
You will get a large number of errors of the form “Cannot find field/set X.” This breaks setups like the one suggested by Looker here: Multiple connections in one model
You can partially fix the problem by duplicating include statements within any given set of Explore definitions. However, I have not been able to find a resolution for the sets issue, and New LookML also seems to break Explores that are defined in one file and extended in another.
We’re unable to complete the transition until the above issues are fixed, and so will be rolling back to old LookML. We eagerly await updates from Looker on the subject.