Kotlin is still in development, beta should be released in following months, according to Kotlin forum, but it is already stable enough to do some real work in it.
Being java programmer for a long time, I always liked it for simplicity of the language itself, stability, large ecosystem of frameworks and libraries, cross-platform support etc...
But sometimes I missed few things that other languages have and Java doesn't. Mostly things that would boost my productivity and make me write less code that is still elegant and readable.
One of those things is passing a function as an argument to another function (feature known as higher order functions or closures). Writing listeners every time you need to use some kind of callback function can be really tedious. If you can pass callback function as an argument to another function/method it can really save time and lines of code. Javascript and Actionscript, languages that I used for some client side development, and that I considered inferior to java in many ways , have this for a long time.
Kotlin, as almost every other modern language has this feature.
Another thing that can save code and time is having default values of method arguments, so passing those arguments can be omitted in method call. This reduces, if not removes, the need for method overloading since it is more flexible way of having different combinations of arguments.
I liked this feature in ActionScript , but Kotlin goes one step further: it supports default argument values, but it also supports passing arguments by name. This means, for example, that you can pass only first and fourth argument, skipping second and third if they have default values. In action script if you want to pass forth argument you also need to pass all arguments before it.
There are lots of other features that make Kotlin code shorter and more elegant then Java code.
Elegance and ease of use is not Kotlin's only advantage over Java, there are also features that make Kotlin more powerful, although with greater power comes more responsibility so in general it requires programmer to be more careful.
To mention just few of them: null-safety, immutability, extension functions, lambdas, inline functions, operator overloading, range expressions... etc.
To see full list of Kotlin features check out docs page and don't miss comparison to Java.
Many modern languages have very similar feature list like Kotlin: Java 8, Scala, Ceylon, Clojure (clojure is little bit different animal since it's Lisp dialect). So why to choose Kotlin?
For Java programmer Kotlin should be the easiest to switch to (except of course Java 8), since one of main goals of Kotlin is to be Java-compatible. This means that Kotlin should provide much better interoperability with Java then Scala, Clojure, Ceylon...
Also, Kotlin compiles as fast as java , that is another goal of Kotlin authors.
Scala is Kotlin's main competitor so it is good to know what are similarities and differences between them.
Since Android apps run on Java and Kotlin is highly interoperable with Java it is possible to write android apps using Kotlin.
Here are some useful links on how to start with it:
For now, using IntelliJ IDEA is the easiest way to develop with Kotlin.
I look forward to make myself more familiar with Kotlin using it on my new android app and I will share my experience here soon.
