I have two internet connections, Zuku broadband (Main) and Safaricom broadband (Alternate), I usually connect my Safaricom broadband to the router and disconect the Zuku one when Zuku goes down since its my main connection problem is: users have to change their Gateway when I do this by disconnecting their connections and reconnecting them again.
I have been toying with the idea of setting up a server using smoothwall so that when I need to make this change all I do is change the gateway on the server and the clients will not have to mess/change their internet connection settings.