I haven't used it but I have used a variety of other treatments. I have used a couple of treatments that are advertised as "100% organic, won't harm inverts" like Ich-Attack
In my opinion, it, and the others similar to it, were an absolute waste of money. They didn't do a single thing to help get rid of ich. But I haven't tried RxP Marine.
I have also tried a couple malachite green treatments which were effective, if they didn't kill your fish in the process. I don't recommend them as they are VERY hard on the fish, and of course stain parts of your tank.
What I have personally had a lot of luck with is Coppersafe
But, do NOT use it in your main tank if you ever plan on having inverts. You should only use it in a quarantine tank. When I had a relatively bad case of Ich in my tank this stuff cleared it up 100% in about 3 days without a single loss. I love the stuff, but use it with caution.