Now every time the dern thing burns up.. "which they seem to do on a steady basis"
Sounds like you may have a power spike or "dirty" power issue. Do you have it on a UPS?
It is dependant upon who your cable supplier is will make a difference whether or not you should buy your own. If it is Time Warner/Road Runner, the bill will be the same either way. The cable modem is part of the service and they will not give you a discount if you supply your own.
One thing to consider; if the company offers 2 different levels of service like Road Runner does, normal=3mbs or fast=6mbs, your modem can sometimes increase your bandwidth to the higher level of service at the lower level price.