I put Google Adsense on Easy Topic Maps two weeks ago. There have been 1,324 impressions and 40 clickthroughs (about 3 by myself), which gives an impressive average clickthrough rate of 3.0% (I’m not supposed to say that) and made me $11.59. Not bad at all. Other people have reported clickthrough rates of 4%, revenues of $100 in a day for a popular blog. Of course, not everybody thinks it’s cool.
I’m not sure I’ll leave it on there, but if you have some very focused and clean (Google declines ‘offensive’ sites, probably based on a crawl) sites, why not?