I'm glad to say that it has been a fairly smooth transition, although some of my code has more than a hint of "let's try this Kotlin feature" as I'm still learning. I regularly use Java, C#, Python and JavaScript for my day job, so many of the ideas in it are familiar.
What I like about it is:
- it's succinct, which is a refreshing change from Java's verbosity
- it's expressive - some of it is reminiscent of LINQ
- it has batteries included, as it can use any Java library
- it doesn't enforce any particular programming paradigm - if all you want is a function then that's fine
- it has new ideas that work well, such as its approach to delegation
If you want to learn it, start with the tutorials, browse the reference and definitely, definitely do the Kotlin koans (I've been doing these on my lunch breaks and they're very good).
Update: this is a very good overview of Kotlin (video + transcript)