I was geting a few bob every now and then. One month my hits got quite a bit higher and they kicked me out!
They informed me that they were invalid clicks and that the cash would be returned to the advertisers. I appealed and they simply said they were invalid clicks and no further contact would be made.
I had nothing to do with the invalid clicks but ended up losing a small source of income, and had to rebuild my website to get rid of the ugly error messages that appeared when they took their ads off.
I am disgusted at Google, but what can anyone do against a giant that is aiming to own the web? My advice is to use them if you wish, but tell visitors not to click them too often!