All I know is that I personally would rather do without than have to click through dozens of popups for one download. If you can do it where it doesn't interfere with the access to the content, fine. When it becomes work to access a free download, you're going to find a huge percentage of folks will stop coming here for content. This, to me, can be a site killer.

I know you have to have revenue, and yeah, I'd love to have more. But you will be walking a very fine line with popup's and click throughs. If they can be kept to an acceptable limit that doesn't interfere with the content, sure. Do what it takes to make the site run. Just keep in mind that the income is based on traffic and if the advertising reaches a point where it drives that traffic away, you've shot yourself in the foot for pie in the sky.

I personally run a popup blocker because of the annoyance facter, but I still hear a ding everytime one attempts to open. Popunder's seem to be getting around the popup blocker however, and this just seem's overly agressive. Of course, these are my own opinions on the matter and everyone will have their own. You'll never keep everyone happy.

 You just need to be clear on what the site goal is going to be about. Is it mainly about the money or is it about the content. When it's about the money, I think you'll find more people abusing the system than if it's to be about the content. Again, that's just my own personal opinion.