i have sello.co - apparently sello is an ebay category in spain and also means stamp/post in spanish. Not the reason I reg'd it but I think it adds value in that sense.
I thought there could be potential when GoDaddy changed their system to register .co as the default instead of .com but that was short lived and the change was reverted.
I'll do .com/.net/.org & .com.au and that's pretty much it.
6 .co which sold for 5 figures or more of which most were bid on at auction when the extension was launched does not equal a strong market. The only people interested in .co are those with a vested interest.