I believe that it really depends on where your traffic is coming from. I have currently added two (iPhone) apps on MobFox, one that has mostly european users, and one that is primarily successful on the Asian Market. I do have their eCPMcontol feature activated for both of these apps.
The first app give me a pretty good fillrate and an eCPM of $3.5, which is, at a 1,2% CTR, a lot higher than AdMobs
For the second app, the fill-rate is pretty poor as they don't seem to have any high CPC ads for Asian users, but I will soon try disabling the ecpm control for this app to see how the "real" fillrate and revenue would look like.
Both apps use admob as a fallback option. I am not using their built-in backfilling feature because it was not yet available when I integrated, so I am manually falling back with my own custom code.
I think it's definitely worth giving them a try if you have traffic from europe or the united states. After all, you can always use your old network to fill when you use the cpm control.