It's not good to put all your eggs into one basket, though. I had a slightly-better-than-breakeven deal going -- I put $300/month into Google Adwords, and I made roughly $400/month from Google Adsense. Then, when Google changed it's Adsense program in September/October, my Adsense rates plummeted. I was actually giving them MORE traffic and MORE clicks, but the average payout went way down suddenly. So I suddenly had $100/month in income from Adsense, but I was still paying $300/month to Adwords. Now I'm in the dull process of diversifying. I'm about to try Yahoo's ad system, I've started using the Digital Point link exchange, and I've begun working with advertisers directly. I should have done all that from the start. -Tony
It's not against the rules, but it is risky. In general it's a good idea to have at least 2 or 3 ad networks you can make use of. Currently I'm using Casale and Fastclick for CPM ads, just in case something happens with Adsense. That way I will still have some income from my site even if AdSense gives any problems.