I have a Horseman 6x9. It's similar to the Horseman 6x12. I liked it in the beginning, but soon realized that I found it annoying that I had to remove the ground glass first before I was able to attach it. So while shooting - if I wanted to do a few different views/angles - I had to constantly remove GG, attach back, shoot, remove back, re-attach GG, etc...
I then bought a Sinar Zoom 2 roll film back. I simply love it. It supports variable formats (6x4.5, 6x6, 6x7, 6x9, 6x12), which can be changed mid-rool, plus it can be inserted between the ground glass and bellows like any other 4x5 holder - it is a bit thicker, but that's not a problem at all. I highly recommend it!