They are fine. The only issue would be if you wanted to flip the site.
.com, .net, and .org's are best for flips.

I believe from a ranking perspective they are treated the same in google as .com

Terry kyle has said that .co domain rank very well so need to worry about extension you can go for it.

Yes, Google treats .co domain as a general purpose top level domain so it should rank well.

.co ranks as well as .com and .net, what really matters in Google's eyes is quality backlinks and relevant/unique content.

.co is fine but it's not cheap.