We've used it here and there, but for the moment we've tended to stick with classic interface protocols as many of our API calls need rather complex request structure setup and often require a bit of work to consolidate and parse the results.

Both of those things lean towards Fetching/Updating protocols and the corresponding classes.

That said, testing has been done largely by using an URLProtocol handler to feed data into the system, and not by creating mock protocol stubs.