Every campaign is different. Every niche topic is different. People's max. bids are different. People's ad writing skills are different.
It's not even like comparing apples and oranges- more like apples and sky scrapers.
One person's .5% CTR may be wonderful for his specific market/campaign, yet another person's 10% CTR may by horrible for her market/campaign.
If I had to estimate an 'average CTR' across the entire AdWords system, however, I'd say it would be in the neighborhood of 2%.
I see tons of advertisers in the double digits, and also many advertisers (who are probably less serious about managing their advertising) at well under 1% CTR.
One thing for sure, at least in my opinion: CTR is very much under the control of individual advertisers, and can be hugely improved by careful keyword selection, careful targeting of keyword to ads, careful attention to writing compelling ads, and so forth.
AWA