Hi Tom.
I use those CPM networks on a few of my sites. They are all "Tier 1" networks (apart from CPX) and they have very strict traffic and quality requirements to get in but their payouts are great. They are "chained" together. ie: the inventory that cannot be filled, gets passed to the next. It's really not worth your while if you have a low traffic site (Less than 5k uniques a day)
Around XMAS time last year I was getting a ridiculous $21 CPM for about 3 weeks. It was a good time to be a publisher.
If you have an entertainment or celebrity site, you can forget about the networks though.
If you need some help, let me know.