I have tried purchasing .co domains and go.co says they are the official .co site so I will use that domain as an example for my post.
I have been domaining for a long time and have visited a lot of .com websites to see who owns the .com of a .ca
The biggest problem I have with .co domains is they keep taking me to the .com website.
For me when I type go.co my browser finishes it off to the .com and I have to go back and backspace the m from com but even that is not enough because it puts the m back in unless I hit the space bar after I type .co
Of course it is doing that because at one time I visited the .com but a lot of clients will visit the .com equivalent of websites they visit so if I was relying on the .co domain I would forever be worried that I was leaking traffic to the .com.
Dot ca does not have this issue and that gives it a leg up over .co for me.