It's only worth it if you make a profit.
The key with AdWords is to not bid on such common keywords as "money", "sex", etc. Find smaller niches within your topic of interest and test your ads until you find one that converts and earns you a profit.
Run an ad for about a week or two and if you are spending more than you are making then either drop the ad or switch the title/contents, landing page, etc.
You can really lose a lot of money on AdWords if you don't monitor your return on investment, but on the other hand if you find a profitable niche all to yourself, you can cash in. I've heard of a lot of people making a living off their Adwords campaigns.
What a lot of people do is join clickbank.com and create campaigns around the various clickbank products. They pay such a high commission that you can afford to experiement a bit with their products. You have to be pretty savvy to pull this off but many are doing it.
I just read Chris McNeeny's Projext X and this guy is filthy rich off AdWords. Some of his techniques are a little ummm.... sketchy and I wouldn't try, but nevertheless... the guy knows his stuff.
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