We have Comcast and bought a Motorola SBG6580. Probably cheaper in the long run over leasing but it's kind of a pile of crap (modem is OK, router and WAP are junk) and when you call for service it's always because of /your/ modem. So in that sense leasing the modem means if you've got a connection problem then you play dumb and blame Comcast.
"Stand your ground. Don't fire unless fired upon, but if they mean to have a war, let it begin here." -- Capt. John Parker