I use usermessages in my gamemode to sync variables between the server and the clients (when the variable changes, of course). And, when a new player joins, all existing variables are sent to him to sync him.
I have read that usermessages are quite expensive, but I don’t really know their effect…
Who do they lag? The client or the server? How do they lag? Do they take long to send (delaying the running code), do they queue themselves to be sent slowly or (worst case scenario) do they fail to send too often?
And finally, are they worse or better than networked variables?