https://www.aventon.com/products/aventon-pace-500-complete-b...
https://ride1up.com/product/city/
https://www.juicedbikes.com/products/crosscurrent-s
Out of those I went with the Aventon, which had a local bike shop selling and supporting it.
50-mile impressions: it's fun; being a cross between a motorcycle and a bike is the fastest way I know to get around Boston at rush hour; it feels safer than biking, because I can almost always go the speed of car traffic and take the whole lane without getting passed; 28mph is handy, even in rush hour city traffic.
It's interesting knowing that these things can be made much more beautifully -- it's still such a young industry that there's lots of room to figure out new standards that will improve things for everyone. It will be neat to see them evolve.