I'm currently a developer of Siena but not since very long. Let me explain why I became a developer on this project?
I went to Siena because I wanted to use Play+GAE and Siena appeared to be a good start for GAE DB and I really wanted to avoid JDO/JPA.
Then, I began to really appreciate Siena for its straightforward, light and easy approach and so simple APIs. It doesn't pretend to be the all-in-one abstraction layer like JDO and the greatest standard DB API like JPA. It really made me think of DB APIs from Python/Ruby and it really fits my point of view: I want a simple DB API which allows me to solve the great majority of my problems and when I have a more complex problem, I will use the lower layer APIs but certainly not an abstraction layer such as hibernate.
The possibility to make my code work on GAE DB or JDBC was also a good aspect. Once again, Siena doesn't pretend to provide exactly the same things in both worlds because SQL and NoSQL are not really compatible (but ORM is neither really compliant to SQL model :) ).
But once again, it is quite practical to be able to rely on the same APIs in several DBs.
Finally, the library is ONE jar and you don't have to retrieve the whole universe to use it.
So, I became progressively a committer on Siena because I wanted to take part of this nice little adventure.
Now siena team is working on a new version keeping the same simple APIs, bringing new interesting features and really improving all the backend code to make it even easier to extend for new DB support.
Siena is a pragmatic API driven by user experiences and that's why I like it ;)
Pascal