I've got one. Only put a couple hundred rounds through it, but it does a pretty good job -- around 800 rpm without a can, 900 rpm with one. BIG difference from stock, which on mine runs around 1,100 w/o a suppressor, 1,200 with.
It runs at a steady RPM, with no surging or change. I haven't shot it enough to find out if it speeds up over time as the spring wears in/out.
I went with a DIRR instead of a slowfire conversion because I wanted to keep the package small -- that's the whole point of an M11/9 to me.