I'm using the client with ZCS 5.0.11 (NE). The server is an Intel Mac running 10.4.x.
When I create a filter on the client, it doesn't do anything--incoming mail is not acted on in any way as far as I can tell. Checking the web interface, I also see that the filter isn't there.
If I create a filter in the web interface, the filter is not reflected in the client. However, a filter that files messages does work. But a filter that forwards messages seems to just delete them unseen, which is particularly troubling.