They generally just work (as long as the component members are also serializable by Kryo):
case class Foo(id: Int, name: String)
// setup
val instantiator = new ScalaKryoInstantiator
instantiator.setRegistrationRequired(false)
val kryo = instantiator.newKryo()
// write
val data = Foo(1,"bob")
val buffer = new Array[Byte](4096]
val output = new Output(buffer)
kryo.writeObject(output, data)
// read
val input = new Input(buffer)
val data2 = kryo.readObject(input,classOf[Foo]).asInstanceOf[Foo]