The JetBrains guys will probably be a little more cautious than I :), but FWIW I've been developing software in Kotlin for a few months now and its been an absolute blast.
Kotlin is a nice simple language thats expressive and concise yet has no magic or surprises so its super simple to read and understand, has a fast compiler, great IDEA plugin and integrates very well with the Java ecosystem and creates nice tiny bytecode with minimal dependencies. kotlin-runtime.jar is currently under 400k, compared to 1.6Mb for guava or 7-9Mb for scala-library for example.
Sure there’s a few compiler bugs you’ll hit along the way (which is true of most non-Java languages on the JVM to be honest :); but I’ve found them quite easy to work around if I hit them so far - there’s always the safety net of writing some Java code if you really have to if you hit some critical bug you can’t work around - though I’ve not had to yet.
The biggest issue today for me is the lack of kotlin runtime & maven plugin in the central maven repo which makes it kinda painful to use things like maven and kotlin right now; but hopefully that’ll be fixed in the next couple of weeks, well before your project starts. (You can work around this by just reusing a local kotlin build in your pom.xml, for example).
I’d maybe wait a little bit though if you need the JS support though; thats a little way from being really done, but its progressing well though - certainly I’d check it out when your project is close to starting.