I think JSON should work nicely with the latest SVN (FPC 2.5.1 and latest Lazarus 0.9.29-30).
Check out the following FPC packages:
...fpc.../packages/fcl-web/examples/jsonrpc/*.*
...fpc.../packages/fcl-web/examples/webdata/*.* for ExtJS
and
...fpc.../packages/fcl-json/examples/*.* for general json handling.
Also, there is a new JSONViewer demo included in Lazarus under ...lazarus.../tools/jsonviewer/ made by Michael Van Canneyt.