The way I designed the api makes it very easy for anybody to add a new protocol, it was just that I have been a big fan of xmlrpc practically from it's inception.
I have used the api a lot, and I would say It's main weakness is in catching errors and returning the correct error code, before they are sent back to the client. I tried to re-use as much of the actual webERP code in the design as I could, but this means that sometimes an html error message is returned which the client tries to interpret as correctly formed xml, fails and spouts a strange error message. This means that debugging errors can be a bit of a black art.
Otherwise it is fine
Tim