It did surprise me to see google ads on amazon.
On the other hand, collectively my sites are earning in excess of $100/month via AdWords. The lion's share of that, about 50%, comes via a travel photography gallery that also feeds one of my cafepress stores. Critical Graphics earns about 5% of the total ad revenue, however, it has the highest clickthru ratio of any of my sites (it just needs to get more visitors).
I'm of two minds.
1. I like the additional revenue, and visitors appear to benefit from the 'service'. I look upon it as if it were affiliate revenue.
2. The ads, unsurprisingly, appear to erode total traffic on most sites, though not all. (If they're clicking on an ad, they probably didn't want to be here anyway.) The ads are very often not as targetted as I'd like. (Probably an indication that my SEO needs some work.) Also, I haven't quite designed templates which will make the ads less unappealing visually. (I'll try toning down the border, or removing it completely.)
It surprises me a bit, actually, that it's the google ads that strike some folks as unprofessional. I'd have thought the overly wide selection of products would have bugged people more. Mousepads? Clocks? Cheap baseball caps? Come fall I'll re-evaluate products and probably cut back to stickers, shirts, buttons and, perhaps, posters. Critical Graphics customers rarely buy anything else.
Cheers,
p.