I have one! Very nice, accurate rifle with the right load. I was the second owner, the first had left the gun hanging on the wall all of its life.
I use a Lee .575 495 grain traditional minie, 50:50 olive oil/beeswax lube and 45-50 grains FFG. Its pretty accurate but the front sight is way too low for anything under 100-150 metres.
I keep meaning to glue a bit of brass shim to the front site to see if I can fix that.
Have not been able to get good performance with a .575 round ball so far, although I suspect using a double patch (0.020 instead of 0.010) helps, but is really hard to load unless you have a mallet!