They cap your bandwidth, according to my friend in SF who uses an Xfinity router in bridge mode behind his own firewall.

Those are the old plans. In the last few months they have rolled nationwide a new set of 4 plans: 300 Mbps, 500 Mbps, 1 Gbps, and 2 Gbps. Upload speeds vary I think depending on whether your area has received the mid split upgrade they have been rolling out for a while. The mid split upgrade allocates more channels for uploads.

These plans all include unlimited data and an Xfinity gateway.

You aren't required to use their gateway, although they will still ship one to you unless you ask them not to. There is no discount for not using it. Unlimited is tied to the account, not the modem or gateway so you still get unlimited if you use your own modem. Same if you use their gateway but put it in bridge mode and supply your own router.

For new customers each of the new plans can be gotten at an introductory discount. The discount price is guaranteed for 1 year or 5 years (your choice, with 1 year generally giving a steeper discount). These are all month to month plans so you aren't locked into a contract.

Existing customers can switch to the new plans. There is no discount but they do get the 1 year or 5 year price guarantee.