.co is a TLD intended for websites hosted in Colombia.
Second, users are habituated to
.com. The missing
m is perturbing, and many people will forget about the fact that instead of accessing a company website, they must go to a website with a Colombia-type name.
This being said, some well known companies, including Google or Twitter, reserved
.co domain names for their use. Actually, it seems that they do it only to protect their customers, but they could also do it for two reasons:
Typing on a mobile device is hard. Typing
t.co is much easier than
twitter.com on a small tactile screen in a subway. Note that if you have to pick between
example.co, the first one is faster to type even on mobile devices.
When there are billions of requests made to the APIs, shorter URIs provide a slight gain of performance and bandwidth usage¹.
Aside those two cases (both having a very limited scope), stick with a
Also, note two things:
If somebody has already a website with
<your company name>.com as the address and you want to create a
<your company name>.co, think about legal consequences. In most countries, having the same name as your concurrent is illegal.
When the users don't remember what is the URI of your website, they will either type the name of your company in Google (most of the cases) or they will type the name of the company, followed by
¹ Still, a developer must be cautious before opting in for short URIs for the API. For example it would be terribly stupid to create short URIs to reduce bandwidth, and at the same time send a 301-type response to the client, forcing it to redirect to the full URI.