I had a pair of old Addidas Stan Smiths. I drilled some holes and made a LOOK riding shoe. It works pretty well. Obviously not as stiff, although if I used it a podiatrist made orthotic I had made it better. If you want the stiffness of a real cycling shoe, buy one. Maybe those wooden shoes the Dutch wear.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.