Mine came in as well today, that was quick!
I briefly tested it out and seemed to run into an issue where my MiSTer doesn't seem to turn on, in my cab, although the JAMMA Expander lights up. It may be because I bought a fully preconfigured MiSTer with addon boards, although I removed them and double checked that I configured the MiSTer itself correctly per the website instrcutions before installing it into the JAMMA Expander. I'm guessing it's either a preconfiguration related thing I need to change, or some other form of user error. I'll find out after tomorrow when I'm off work and have more time to tinker.
Most likely I'll buy a bare DE10 Nano and RAM just for the JAMMA Expander soon. I didn't expect to enjoy the MiSTer on it's own as much as I do, especially with the analog add on board. It feels so much simpler and faster than dealing with a Pi or other emulation based mini PC, particularly the menu and controller config.