I'm not a Premium member so I probably won't be much help, but I think paying for the membership is probably worth it. If you stick with a regular membership and upload a chapter a month, you run the risk of critiquers not having read your previous chapters and therefore not understanding what's going on. You don't want to get a load of critiques that are actually irrelevant because the critique in question hasn't read the first few chapters. It's also better if you get the same people critiquing each new chapter because then they can see how the novel is developing as a whole.
Premium members get to have their own forum so they can invite any of their CC friends to join.
They can also create their own queue and invite people to them.
They get access to the buddylist feature. This means they get notified whenever one of their friends posts a story. So if you had, say ten buddies on your list, they would notified every time you posted a chapter. It saves them having to find it in a queue.
As I said, I'm not a Premium member myself (I have considered it, but I simply don't have the money to spare at the moment :( ) but it does seem like a good idea if you're trying to get feedback on a novel.