I'm calling the banner ad experiment to a halt. I give it a while and various experiments but here is the deal according to me.
On Ad Placement
1. Network-wide banner ads suck. They are like sending everyone on the planet junk mail - it has some return, but you are essentially polluting. <1% can make money but it is pollution.
2. Domain targeting makes a difference. Your banner ads will consistently perform better on some sites than others. You have to experiment to find the right ones.
3. Search advertising is great, but it only works if you can offer something in a language that people are looking for on a search engine. (Sometimes people don't know what they want, or don't look for it on a search engine.)
On Copy
4. Some of my ads had the linkspank logo; others were clever and obscure; others were in-your-face. The best combination seemed to be "in-your-face, but slightly obscure."
5. White text on black background wins. I learned this in Ogilvy on Advertising and I recommend the book.
Summary
6. It's too expensive, and doesn't feel good, compared to other methods of promotion - like the contests.
What does it all mean to you?
I don't know, but I'll offer an opinion in an area that is interesting to many people right now - mobile phone advertising -- and especially mobile-phone promotions and contests.
Google has launched "Adsense for Mobile" -- namely, banner ads for mobile, so you want to know, should I be putting my banner ads on people's phones?
I would say, try going to the mirror and saying to yourself, "I run a junk mail business." If that sounds accurate, go for it! ;-)
People also have a blithe notion these days that by offering customers the opportunity to win something, you are immediately their friend and they won't mind constant contact from you. (E.g., constant text messages.) My dos centavos is that you should keep in mind that a good chunk of people - possibly 100% of your target market - will assume that it will be impossible for them to win, even if it is not that difficult (I learned this in the case of linkspank).
AdWords/the search context is just a way way better way to reach people, if it makes sense for your medium. Otherwise, go for some PR stunts and work on using your consumer evangelists - they WANT to receive your text messages. :-)