I have Comcast in Lexington MA and there are no hidden fees, assuming you just have internet-only service. (In other words, the price listed on their web site is exactly what I pay every month, and not a penny different.) I think if you also have TV or phone service through them then there may be sales tax, "franchise fees", etc. which maybe you could consider "hidden." But none of those exist on internet-only plans, as far as I can tell. It could be that that varies by town.
In all honesty, I'd probably just play the two off each other. Call up RCN and threaten to cancel, saying you could get X price from Comcast if you switch. They'll likely just match it.