Hi Everyone, thanks for your helpful responses.
I signed up with Sonic again for the AT&T-provided service. I live in the middle of SF and AT&T had run fiber in the overhead lines, so the installer had to call a guy in a motorized bucket to get the fiber installed. For some reason he hadn't planned for this, but by the end of the day they got it done.
TLDR version - everything is working great, speeds are pretty much 1 GB/s in both directions. It's only been 2 days but so far so good. I'm almost finished watching everything on Netflix and 100x speed.
Longer version - the fiber on the street is heavily armored and the installers can treat it pretty roughly, which is good, In my case meant they could throw it over an overgrown tree and hook it onto the side of my house before stringing it through a hole in the wall of my garage. The actual fiber strand is very delicate, so they install it inside where it's somewhat protected. The fiber goes to a small box that converts the optical signal to an electrical signal, and that goes to the (AT&T supplied) router via a Cat 5 or 6 cable. You need a straight shot of Cat 5 or 6 (both support GB bandwidth - in my case I've got Cat 5 and it worked fine) from the optical line terminal (OLT) to wherever you want to put the router. This is something to think about - some combination of fiber and Cat 5/6 needs to get to your router.
As another poster mentioned, AT&T supplies their own router. You need to use the AT&T router, but they give you all the passwords to configure it. Since I already had a router and a wireless setup (soaked in blood and tears to get around years of struggling with super-dense SF radio noise...) I turned off the WiFi on the AT&T router and configured it for IP passthrough to my existing router, which is actually the only thing plugged into the AT&T router.
I'm happy so far and am looking forward to being a Sonic customer.