My experience has been that Comcast's service is MUCH better than RCN. Much more consistent cable modem service, fewer billing problems, and better customer service. I had all kinds of problems with RCN, both in Somerville and in New Jersey. I've had Comcast (actually, MediaOne, then AT&T, then Comcast, as the corporate takeovers have rolled along) in Somerville and Medford and have never had a major problem.
It's also worth mentioning that I've heard from a number of people that DSL service from Verizon is very unreliable and can take a long time to get installed.
But they're pricey! They're really stingy with the "on screen guide", don't have well priced bundles. How is their Usenet feed?
Haven't compared prices since we don't have RCN in Medford, but it might be a case of "you get what you pay for." I will say that you can do quite well without paying lots of extra money for Comcast's digital cable packages. Analog cable works just fine. I rarely use Comcast's newsserver, so it's hard for me to comment on it. However, if you want me to check for availability or retention time of particular groups, let me know.
I use Comcast and have no complaints, but no experience with their Usenet either.