I don’t believe you often see Google AdWords representatives talking about click-through rates. But in a Google AdWords Help thread, I spotted AdWordsPro, an official Google representative, taling about averages.

In short, he said that a good average to aim for in an ads click through rate (CTR) would be 2%. But he/she has seen CTRs in the double digits for campaigns that are constantly tuned and tweaked by advertisers. Let me quote AdWordsPro:

I have heard of incredibly high CTRs, especially for Google ads with thongs in them. But outside of that, you know what to generally aim for.

Forum discussion at Google AdWords Help.