You should always purchase your own modem. Don't buy it from your provider either, don't even let them give you one. Always do your research on the modem, even if you're not going to use it's full capability... in the future you may.
Motorola owns. If you get the best model (usually for around $100) and pair it with a great N-600+ or AC router (Netgear owns here IMO) you can't go wrong.
So side from saving money why should you always purchase your own? Especially with Comcast...
Because Comcast USES their customers who have leased modems. If you have one of Comcast's modem/router combos you're not only broadcasting your own private wifi but also Comcast's public wifi network. People can hop on your modem and draw your bandwidth. They don't access the Internet from your IP but they do draw bandwidth from your modem. You can now call 1-800-XFINITY or visit http://customer.comcast.com/ to disable this "feature". But in my experience, even without that... their modems suck. They're huge and you don't want to combine your router and modem. I change my routers out more often than my modem needs to be changed...
More info > Comcast sued by customers for turning routers into public hotspots
You can get much better supported routers in the open market that will give you a much richer feature set and technologies behind them. IMHO it's always a dumb move to get modems and routers from your provider. ESPECIALLY if they're a combined unit. YUCK! No thanks. If they gave one to me... I wouldn't even sell it or give it away... I'd disassemble it for fun, see if any parts could be salvaged. Get that public wifi out of people's houses! It's a disgrace and they should be ashamed for doing it without really telling people. Hey Comcast, install that crap out in your pedestals at the street and pay for your own electricity!
