It's great to hear that the test results were so positive.
That's the risk Google took when it went for market share at the expense of quality control. Google wanted ubiquity, a la Amazon.com, and soon it will have to make some decisions about all the dodginess that escaped from Pandora's Box. :-)
Never mind the clickthrough rate--IMHO, the correct formula is "high-quality content site = higher-quality, better-qualified leads."
Take one of my favorite examples: Luxury cruises that cost $700+ per day. The ideal clicker for such an ad is someone who's just read a cruise review and knows that a two-week Platinum Cruise will cost ten grand. Thanks to the review site's high-quality content, Joe SixPack (the guy who's looking for a $100-a-day cruise) will understand that a Platinum Cruise isn't for him.
That's what might be called a not-so-glittering generality. :-) For what it's worth, I continue to see many of the same travel advertisers in my niche week after week, month after month, so I can only assume that AdSense is working for them.
Maybe your industry is different. Or maybe you should be following Shak's example by spending more time or money on "creative." Not long ago, somebody complained of too many clickthroughs by freebie seekers who were taking advantage of a free offer. That was a perfect example of an advertiser who needed a new copywriting strategy. If you simply want names and e-mail addresses, offer something free. If you want prospects who may actually buy something, you need to prequalify leads by using appropriate ad copy. To continue with my hypothetical cruising example, if you're selling luxury cruises, you can't just say "Discounts on Platinum Cruises." You need to make it clear that Platinum Cruises are for the Bulgari-and-Baccarat set--whether you say so directly or via code phrases like "six-star cruising" or "Platinum Cruises at Sterling Silver prices."
AdSense obviously is working for some advertisers. Perhaps you should be thinking of how to make it work for you.