I clicked on one by mistake once. Now that I know what they are, I avoid them. From what I hear, the heuristics are faulty somehow and they really don't deliver relevant ads.

I use Google Adsense on my mainstream sites and yes, it can generate good revenue. I know alot of webmasters that use them and a few that are making over 200 bucks a day from them. One trick to getting better CTR's is to place the ads into your content so that the ads look like it is apart of your content and not just an ad, no borders, same colors and fonts. Also most people start looking at a page from top left down. That would be a place to put them.

A good webmaster resource is www.mainstreamwebmasters.com/ you can find many useful articles on the subject.
Good luck with it and never give up!