I personally think .com has reached its ceiling. Domain realtors have booked anything thats worthwhile and unless you plan to shell out 4-6 figure sum, you wont get the one you are after. .net is almost in similar lines and .org is not that popular... how many commercial or marketing names with .org do you see popping up in google searches.
.co is a domain name first authorized for colombia but now over the years its handling has changed and theres a new promoter. There is mixed feeling within the industry as to its success or failure. But no one can dismiss the fact that it can easily stand for company & corporation and its primarily being promoted that way. Currently there no other TLD that can give resemblence to the term company.
From what i can see if a key domain name is taken and parked by a squatter and you have the option to get a .co you should definitely go for it. Heres an intersting article if you are interested in speculating on .co
Finally wether you take a .co or not, your success depends on the name taken and what exactly you are going to do with it. Expecting to book a name just for sale is not a rigth move ( It may work or it may not work ) but planning to develop a website with that name popularize it and sell it would be a planned approach.
1 year down the lane we will know if .co was a boom or a bust.
