So far, it was mainly being used for the linuxppc and netfilter project with mixed results. I guess in both projects, in the end, nobody raelly maintained the patch status and stuff just ended up to get stale.
Now I've started an patchwork installation for OpenMoko, at least for the most common public mailinglists. So why do I think patchwork will be used more productively and receive better maintenance? Well, firstly, the number of patches on those lists is still quite small. Secondly, the number of people looking into reviewing those patches, within their primary paid-for working time, is relatively large.
I really believe that patchwork can provide an enormous benefit to a project. Let's hope we manage to use it correctly :)