Jon Udell at InfoWorld has been tinkering with the first GData API (Google Calendar's), check out what he has to say:
"Along with HTTP Basic, Digest, and a couple of others, this Python HTTP library [httplib2] will now handle Google-style authentication. That's really the only tricky thing about using Google Calendar's API. Everything else is URLs and Atom entries. There are Java and C# wrappers for this stuff, but I'm having a ball just using Python's interactive mode to explore the Calendar API. Among the things I can easily do: search for entries matching dentist, search for entries after June 10, receive the results of any query as an Atom or RSS feed..."

"Most discussion of Gcal has (appropriately) focused on its user interface, which puts many a conventional fat client to shame in terms of both its responsiveness and its ease of use. From my perspective, though, what matters equally is an API that's powerful, flexible, and easy to use."