I currently use the camera with the R32, R40, S43, R90 and S150. The back i use is a IQ180 and that is the reason why I'm probably selling the S43, just to much Colorcast. I really like the camera and can't complain. I love the lens shade system, which I personally think is a Joke on the Alpas, or should i say none existend ? Especially on the Rodenstock lenses a good lens shade is important. I will probably write more on the subject once i'm back from Asia and have some more time.
I think both the Alpa and Arca are great systems. In the end I think currently the Arca is the better option, if one does not plan to get a camera like the Alpa TC. This is where Alpa really shines, being able to use the same lenses on a camera like the Alpa Max and the Alpa TC. However, I still have the Phase One 645 with some great lenses so i don't need a "tech" camera for this prupose.