Another DIR-615 user here (supplied by VirginMedia back in Lancashire), only without DD-WRT though, not braved modifying the router yet to try it...
It's connected to some cheap "Technicolor" brand modem (apparently formerly Thomson brand, which was formerly Alcatel), but the modem's acting as the DHCP server and the DIR-615 as a wired & wireless hub, though with all the additional hardware I keep acquiring, I may need to upgrade to a router with a few more wired ports...
Makes me wonder though, if I replaced the modem with something more well-known whether down & upload speeds would improve or not...