This is my second hydraulic forming die (Whidden was my first) built by my gunsmith since I have all the reamers. I like it, but I understand it is not for everybody, but it is simply another method of pre-forming a brass. Yes, I do the COW method too. It boils down to personal preference. I could have done the COW method (and still can) but opted for the hydraulic forming die. The one advantage of using a hydraulic forming die is you save primers - we all know primers are a premium these days. I do not know if you have seen my COW with my .30 Gibbs.
From L-R: .35 Whelen virgin brass, after re-sizing, fire-formed with 11g WSF (COW method), and after re-sizing.