What are the factors that influence your decision to serve any kind of pop advertising (e.g., pop-up, pop-under, pop-in, etc.) on a particular web site?

I've done just a little experimenting with pop campaigns ranging from a few days to a few weeks, and so far I've found that these factors are important:

1. Expectations of user audience. For example, if you are offering a free service, how much advertising are they willing to put up with from pop ads to still get to the content or service your site provides.
2. Ability to shut down illegal malware ads running any kind of adware, spyware, viruses, etc.
3. Overall ability to manage frequency of how many pop ads are served for a given time frame (per browser session, per 12-hour period, per day, etc.).
4. Revenue comparisons. So far, I've seen that no other CPM comes even close to that paid by pop ads, which pretty much explains why I'm even bothering to consider serving them over the long term.

I'm new to the pluses and minuses of using pop ads, so any comments or suggestions are really appreciated!