GPGnet (client for SupCom and Forged Alliance) is the best multiplayer-client I ever knew, if you look at functionalities: Live-replay (watch replays of games that are played right now, which is great for tournaments), tournament support, ranked ladder, map vault, mod vault, replay vault, player profiles, friendlist, great clan-support.
In the early beta GPGnet really had some kind of problems, but with supcom being released, everything worked fine (still, most players need to setup their router with port-forwarding, but well with my new router, i did not have to set up anything).
The bad side of GPGnet is its instability. But since we had a hardware upgrade for our SupCom servers last week I have the feeling its better again. 
I for myself always wished that Demigod would use GPGnet too, and I really hope SupCom 2 will be over GPGnet. But well... I was disappointed about Demigod. I really would have appreciated it to be handled via GPGnet. 