Usually, not. You get the full bandwidth of your DSL line when you need it, the hotspot either only gets what remains (aka is in idle) or has its own PPPoE session that lives outside of your contractual bandwidth limit (obviously that one only works when the line bandwidth is greater than the bandwidth you're paying for).
On that one, you login with your Xfinity account.
I don’t feel that it works especially well. They make it difficult to log into different hot spots. They have a “remember this device” thing, that doesn’t work very well.
To be fair, it's not a bad idea... if it was open for everyone (helping out one another) and not just benefiting their customers (helping the company extend its reach). As much as i hate Free ISP here in france, for some time when it was impossible for me to get connected to xDSL (but my neighbor was!) FreeWifi really helped me stay connected to the Internet.
Still, fuck them for calling it FreeWifi (wtf, a corporation is like the exact opposite of freedom). If you're looking for truly free wifi communities, i strongly recommend to check out Freifunk they've been doing that for a long time building OpenWRT images for popular consumer modem/routers in Germany.
Looking at your Xfinity, coverage across all of SF..