For the most part, I've been quite impressed by AdSense relevancy. For a while now, I've had nice colorful Family Guy ads on my home page. When I wrote about my Orange Chicken incident, a commenter pointed out that I hilariously had ads for Orange Chicken on my blog.
But ad-sense can backfire as well. I had one post where I was critical of the Republican party and I noticed that Google was showing ads seeking fund raisers for the GOP. I laughed out loud and figured, hey, that's a good way to keep my blog balanced. Everytime I criticize the Republican party, my ads will balance that by advertising for them. I'll criticize the ineffective Democrats as well so that this post gets ads for Libertarians or the Green party.
UPDATE: Looks like Rory's noticing how eerily relevant AdSense can be.