I'd say suspicion is always wanted with things like this. If you know of other services you would like to see data transfer to/from, please let us know, we want this be open to everyone, big and small, and are looking for suggestions.
FWIW, the team that is building this at Google is the same team that builds Takeout, so we've have been trying to give users useful tools for leaving Google a while now. We think giving users the ability to directly move data to a new service provider is the next evolution of the Takeout ethos about not locking users in.
I'd lije to be able to transfer all of my maps data. That is the places I have marked by both name, and location as well as tag, reviews, and photos.
places I could exchange this data with include OSM, Apple Maps, Yelp, etc...
How welcoming of contributors is the project?
Re: Java, ya its in the roadmap that the adapters should be language agnostic. Forcing them all into one language, regardless of what it is, is kind of lame.
Check out https://github.com/google/data-transfer-project/blob/master/... for ways to stay in contact with us and start contributing.
I will try to refrain from criticizing the project until I am more familiar. The next time I interact with this project will be on github, my username there is the same as it is here.