Sadly, they hold the most credibility. Everyone's heard of .com, specifically the casual web users or less tech savvy. I tried .co once, but when I passed a mention of the domain on to someone else, what did they type in? That's right, .com.

Locally (.co.uk for me) there's many more available, but that makes it harder to target a global audience, and also as above, a lot of people tend to try .com first, potentially introducing them to a competitor.