Having been to Colon I can say that this city is quite something- a slum - but intersting. Watch your back.As for the the "Free Zone" it was intended originally for wholesalers from surrounding countries and more recently sales to individuals off cruise ships. At hotels you will run into a lot of middle eastern traders who work out of South American countries.
As a motorcycle tourist, and not committed to the buying of overpriced stuff I think it is highly overrated as to how much one can" save" on so-called designer fashions electronics and junk. How can you be sure you are not getting knock-offs, pirateria, or refurbished items? Then , how much does the shipping - home of such cost , and what surprises of import taxes etc. await you once you do get it home? Just because Panama waives taxes does not mean you are home-free.