Kryo is a very new and interesting Java serialization library, and one of the fastest in the thrift-protobuf benchmark. If you've used Kryo, has it already reached enough maturity to try it out in production code?
Update (10/27/2010): We're using Kryo, though not yet in production. See my answer below for details.
Update (3/9/2011): Updating to the latest Jackson and Kryo libraries shows that Jackson's binary Smile serialization is pretty competitive.
There is a bug report and a discussion thread. The DateSerializer that comes with Kryo is slightly more efficient size-wise than the SimpleSerializer implementation posted on SO because it uses LongSerializer optimized for positive values.
Edit: I forgot to answer the original question. I believe Kryo is used in at least a few production systems. There is mention of it in this article, Jive SBS cache redesign: Part 3. In the Destroy All Humans project, Kryo is used to communicate with an Android phone that serves as a robot brain (video here).
Not a direct answer, but you might browse the Kryo source and/or javadocs. Check out the read* and write* methods on the Kryo class, then look at the Serializer class. This is really the core of the library.