I like the "mini velo" 20" wheel road bikes a lot. 20" wheels are rough to ride on, but with high quality recumbent tires, there are advantages:

Small wheels can be very light weight, and with the rotating mass closer to the hub, can speed up faster. The small wheel also allows a rider to make use of a gappier cassette or freewheel. A 5 speed freewheel with a massive single chainwheel will probably still be pleasant and efficient.

I think they look like a hell of a criterium bike. I want.

I have seen some people put 20" wheels on standard road bikes, but the BB is very low to the ground, and the bikes had to be modified to move the brake calipers.