I've been with fullscreen since October, with the Tiny Galaxy subnet. It has up sides and down sides like any network...unless you're a huge channel (100,000+ subs).
I originally signed because of the Gorilla campaigns, which at times are just flat out silly and i would never do, others i don't mind doing so much, and the payout on them are pretty big, for me... Also, and mainly, for protection from adsense... YouTube has a funny way of denying videos for monetization for the dumbest of things, get denied too many times and it's lights out for your adsense account! Which for me would be devastating since i earn a living from my YouTube channel.
Partner support can be VERY hit and miss. I don't have very fond feelings for the Manager of Tiny Galaxy personally, i feel that he severely lacks in common YouTube knowledge and doesn't understand even half of what he should! This has been proven to me several times in emails between him and i, as well as a lengthy skype conversation between him and one of my close friends who is also partnered with Tiny Galaxy. In short, i have to really dumb down my emails to him to get even some what of a desired response.
As far as earnings, although my CPM is generally higher than it has been with other networks, there is pretty much no (wanted) promotion at all. There is the option to opt in to network promotions, where you have to allow them to like and favourite videos using your channel, which i am NOT a fan of. I run an Android based channel, and having iPhone videos liked and favourited and having my subscribers see that is a stoning waiting to happen...so i opted out of that lickity split.
The Gorilla campaigns are still lacking in relevancy, as far as tech is concerned.. And most of the Gorilla campaigns are pretty much only promoting other Youtube channels, which pisses me off when i'm told by the head of Gorilla that their sales team is hard at work... Well, if they were hard at work, we would have sponsored ads like other network channels do (revision 3 and so on) So the only Gorilla campaigns i'm willing to do are the $25 cpm ones, which of course, are YouTube channel promotions.
But again, for me it's all about adsense protection, and protection from YouTube flagging my videos with copyright claims. Yes that's happened in the past, when it was just me sitting in front of the camera talking, no music, graphics or anything, LOL.
I will say though, that once my contract is up, i will be looking for other options. But fullscreen hasn't been all that bad in the grand scheme of things. If i can't find another network with partner and network managers that are more knowledgable about YouTube than those at fullscreen, i'll be sticking around, but at the moment, talking with the network managers makes me want to put my head through a wall, based on the lack of very basic knowledge that they SHOULD know, especially if i know it!
Just my 2 cents