I am in the process of RMAing a FC5v2 right now and so far the process has been sluggish at best.
I emailed them all of the necessary information and they sent me a confirmation email on April 22nd. They said that they would do the RMA as soon as possible. After not hearing back from them in almost a month, I sent them an email on May 15th asking about the status of my RMA. They responded on May 16th with an email saying that the RMA is "on the way but with no tracking number" even though I have never shipped them my defective FC5.
I'm starting to have doubts about the effectiveness of their RMA service...
Anyways, the problem with mine is that the second knob does not work unless pressure is applied to the knob. When there is no pressure applied, the first knob controls both the first and second fans. Here is a video of it in action: