Unrelated domain keywords in a .com are still easier to rank than bottom tier tlds like .co.
I personally would never use them on a serious project with real, repeat visitors, but you can get some nice looking domains. Of course, you can't escape the fact that .co looks like a .com typo. There's also no .co button on the iPhone.
.CO domains should have been a selling point when signing up for a .COM and they should have been given away for FREE to whoever registered that .COM to account for typos. Of course, they could have just sold it for $1 more or something. But not linking .COM and .CO domains is a big mistake on their part. Maybe it'll work, maybe it won't. But they could have made a hell of a lot more cash by offering .CO domains for $1 or two per year to existing customers that owned .COM domanis. Stupid move on their part.