My partner and I purchased a forum, very similar to this one actually, a while back. It was a completely free forum driven by Invision Power Board or IPB software. We had 125,000 members, with between 400 - 1000 online at any given time. The subject was super-ultra-high-risk investing (read: overseas ponzis, and other generally stupid behavior).

Anyway, point being... AdSense generated about 3/4 of our revenue for quite some time. Our average monthly deposits from Google were approximately $30,000. We also brought in another $10,000 or so from ad networks, like Tribal Fusion. And another $10 - $20k from direct banner sales and supporter upgrades.

I no longer own the site... the whole story is probably a good candidate for the "failure" section here. A very long and frustrating story.

But it's a good 1st hand example of having good success with AdSense.

I know of a few other sites that do far greater even, such as the Weblog Inc. family of sites. Some speculation and real data about their AdSense revenues was thrown around a while back when they sold out to AOL for a supposed $25 Mil (I believe 2005-2006). There was tons of speculation about AdSense checks totaling over $1,000,000 / year for the network of blogs. I honestly wouldn't doubt it, knowing what I've experienced and comparing traffic counts.