Everybody wants to hate on my Cro-Magnon brothers, but New York Magazine has an article online in defense of it's survival as a weekly series. Critics are absolutely destroying it, but I think those scores are more indicative of all the horrible buzz leading up to the premier than people actually watching the episodes that have been broadcast. I've seen them all and, honestly, its better than 90% of what passes for network comedy these days. It's nothing revolutionary, but the scripts are tight and the characters are likeable. One of my favorite aspects of the show is the blandness of the three caveguys that are the show's main focus. They're written as total polo-wearing, racquetball playing whiteboys, who happen to still be at odds with society in general, and subject to racial stereotypes and bigotry because of their hairy beast-man appearance. The humor isn't as broad as you would expect, and the situations they find themselves in our generally interesting. Again, nothing outside of the normal sitcom fare we expect. Just well executed and engaging, which is more than you can say about a lot of shit out there.
Also people were having a fit that the 'original' caveman from the Geico ads wasn't in the show, but that's not the case. He's a supporting character named Maurice and has a number of funny scenes and one-liners.
So there you have it. The show is above average and the ratings are serviceable enough to warrant an order for a full season. I don't know how sustainable the premise is, but it deserves a fair shake.