I've had my own frustration with it for a good year. Since I don't care about multiplayer generally, it doesn't bother me very much, but it's just wrong, and even more wrong that no one steps up to make it go away. I can almost never connect to game servers. I've tried opening specific ports on the router, and even DMZ'ing the 360's IP address. One out of, oh, 10 times it connects. The rest of the time, attempts result in a frozen screen until a timeout minutes later (or an exit to the dashboard in disgust). Last night, replying to an invite to multiplayer in Burnout Paradise ended up with me having to exit the game entirely. The Start and Back buttons stopped responding. Is that weird or what? Turns out I was visible to my Friends list, and was able to get the invite, but I was not connected to the game servers. That I guess sent the game to some altered reality, and eventually to one of those interminably long waits on the "connecting to servers" screen.
No one seems to be able to answer this, and all I ever get in response to official queries about it is the runaround. "Check with the other guys" or "it must be your fault", basically is what the answers boil down to.