I have been using ValueClick for a couple of years, and they do indeed pay reliably. However, given the low clickthrough rates on my site, the CPM based plans are intriguing, to say the least.
I did sign up with Amazing Media because it appeared that I would get about six times the revenue from AM as from ValueClick. I started by installing the Amazing Media code on a few of my most high-traffic pages (my site has over 10,000 pages). Immediately, it appeared from AM's stats that I was making as much from those few pages as ValueClick was paying for the entire site.
But then they suddenly cut their rates in half (backdated to December 1st, as someone else pointed out), and still haven't put earnings totals back on the publisher stats page. Moreover, my December stats disappeared, and they never answered my emails asking various questions.
I have always had the sense that CPM based ad networks were flaky, and that clickthrough based networks were solid, and mabe this is just more proof of that. Sigh.