Check out suneido.
I made a fairly complicated GIS app as an experiment with it some years ago (database, complex gui, reports, client/server). It was a pleasant experience (apart from some documentation issues...) and I became productive with it very fast.
I don't use it anymore mainly because:
- it's not really general purpose
- it's not cross platform (windows only)
- I decided to stop exploring exotic
technologies and specialize in something
more mainstream.
