Honestly, I've read around about them so much that 90% of the time they won't help, while 10% of the time they actually do help. Most networks are really looking to simply make money for themselves, meaning they actually don't really care about you unless you're giving them money. If you sign a contract for them that's a) auto-renewel and b) every year, you might suddenly have a popular video overtime and instead of getting the money yourself (which they probably didn't even help the video get popular), they'll take a portion of it!
I've watched many Youtubers talking about networks and whether you should join one and I think the consensus is think REALLY hard if you need them, because you probably don't. Most popular Youtubers didn't get popular by joining networks, rather they most likely joined them AFTER their popularity hit because they realized the network had assets that they needed. If you are considering, be sure to read the contract VERY carefully and ask questions.
If you want a really good video about MCNs, go to Glame&Gore's channel. She had a series after she won a NYX Cosmetics Contest where she talked about starting on Youtube and talks about MCNs and pretty much expands on what I said.
