We use it exclusively on the server side at Verizon Labs, and it's quite beloved by all. We lean hard on the functional side of things and make heavy use of what some people call 'advanced features', but because of our micro services infrastructure even the teams new to Scala can be productive in a less functional style, and refactor later to be more idiomatic.
I do think large teams need to have enough language experts to help from making some bad design decisions, but I have plenty of productive coworkers who are new to the language and learning as they go.
I really don't agree about the 'maintainability fears' or 'write only' language criticisms. 'Bad' Scala is still so much better than bad java, and good scala is phenomenal to work with. Because of the type system, I have a high level of confidence when working with unfamiliar code and making minor changes.
Smart folks with no exposure to functional programming are picking up the 'advanced' features very quickly with the right training and guidance. I would not recommend it to a team who has on one with any experience in the language, but if a team can hire those who have experience with the language, even if it's just a fraction of the overall team, I'd be confident they could succeed.
There are real downsides:
* slow compile times
* IDEs can struggle with advanced libraries (Scalaz)
* some language features are 'sharp' (implicit conversions)
* finding developers who know scala
We're overcoming the first one by the fact that we're already a microservices shop.
The IDE issue doesn't affect me, I'm on Vim, many coworkers on Sublime/Ecmacs.
If you go crazy with implicit conversions it can cause issues, but again we try to mitigate this with some training and having enough advanced users of the language/code reviews to show everyone how to relegate using conversions for syntactic sugar or typeclass conversions.