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.
"Silent enim leges inter arma." -- Cicero