a. absolute lowest latency I've seen to a major IX point in seattle is about 15.8ms. More often it's 22-23ms. On a longer term, averages between 20-30ms. That's for the combined total of four 550km paths through space plus terrestrial latency from their earth station to seattle.
Keep in mind that on something like a DOCSIS3 cable coax last mile segment can have a minimum latency of 12-16ms from just the cablemodem to the local CMTS in the neighborhood, so that's actually really good. It's not as good as fiber but it's significantly better than many other options. One has to account for the need for the antenna/modem to modulate and apply FEC on the Tx at both my CPE antenna and the spacex earth station on the other end (at least a few ms).
b. no ipv6 yet. They have people who know what they are doing for neteng so I am sure it is planned.
c. all IPs are in cgnat IP space right now. Nobody is getting a discrete dhcp-assigned public ipv4 /32 yet. You can hook up whatever router you want, anything you plug into the PoE injector that is an ordinary 1000BaseT 1500 mtu DHCP client will be given an address. The router they hand out is a very bare bones 802.11ac 2x2 MIMO thing that is based on openwrt.
restrictions, you can't move it from your designated service location. That's part of the beta TOS.
Using it, it is a real 150 Mbps down x 16.5 Mbps up. I have put non technical guest users on it doing various things and from their perspective it cannot be distinguished from the separate terrestrial connection at the same location.