For an often searched 1-2 word terms, CTRs can average from 2-20% depending on the industry, keywords, competitors, ads, etc.
Although, AdWords has gained user trust in this area. The CTRs for the new premiums with AdWords is definately higher in many areas than it ever was with the old CPM buys on G.
I find it varies a lot more if there are two horizontal ad's - but I would say my ad's tend to average 25% on top, and 15% in second.
However, I am bidding on quite specific keywords a lot of the time - for more generic terms I would expect the CTR to be a lot lower.
So a lot of people use the premium positions then. Hmmm....
"Use" would be the wrong word as one cannot elect to be in the premium spot.
You can seek to work your way there, or you can be fortunate and just find yourself at the top; either way good, well-written copy in an ad that clicks to a highly relevant page/site, corroborated by a high CTR (of an undetermined %), other factors relative to CPC and undoubtedly a host of other elements will see the ad under the scrutiny of a human review.
Should/when it passes muster, your ad will appear in the top spot...
There are many threads to be found (using the search facility) that will provide useful insights and the opinions of others better experienced than I as to how best to reach, and stay in, the Premium slot...
Or you hope that eWhisper will conveniently come along yet again and point out the best threads on this subject area... ;-)
Syzygy