If you know you are going to use Comcast for a year or more, it is cheaper (or break-even) to buy your own modem. Amazon has the Motorola surfboards (SB6120 and SB6121) for $75 and $85, respectively (vs. $7/mo *12mo=$84).
I have a Clear4G mobile box sitting right here next to my keyboard. I have this as a backup since my TWC connection completely crapped out for 3 days and I work remote. While it is easy to install (walk to BestBuy and purchase, turn on) I've found the speeds to be highly variable, sometimes losing bandwidth all together. I live in an area zoned for "Excellent coverage" so not sure what the issue is.
BTW, like the OP, I have expensive Comcast service because I have no other effective choice, and they just sent me a box and let me set it up too.