I've had such a terrible experience with AdSense. My account was locked with $10k in the account and the only explanation they gave was "click fraud". Had I been committing click fraud I would have shrugged it off and moved on BUT there was nothing fraudulent going on at all. The account got frozen and the money I'd earned was lost. AdSense certainly acts like a monopoly and it's extremely frustrating. Now I cannot even use AdSense for any future projects which unless I'm running a high-traffic site severely limits my advertising prospects.