I'm taking a cautious approach (as with any new libraries), but so far it's been in a couple of small projects in production and working well.

It is certainly far easier to write new implementations with Akka.NET in mind. But there are a couple of strategies that I found useful in retrofit scenarios:

1) Use adapter / proxy patterns (with DI / IoC). A bit tedious, but easy to back away from.

2) Use async more. It does force you to adopt a certain mental model, which helps make the transition to Akka a bit easier (rather than returning Task<T>.Result everywhere from Akka actors).

I'm starting to use this in production and I can tell it's solid. So far I didn't find any breaking bugs. The team in charge is also very quick to respond to any question.

The project is based on Akka for JVM which has 5 years or so and is rock solid and very widely deployed, so you can rest assured the concepts are proven. The documentation still needs some work, but there are plenty of documentation for Akka for java/scala and most things have the same name so it's quite easy to find resources about it.