Which Jack? Jack1 or Jack2? [1]
My experience with JACK seems very similar to everyone else's early experience with PulseAudio, really buggy, lots of audio pops (small buffers) or latency (big buffers).
One of my most technically adept friends has spent a long time trying to setup a basic debian install with JACK working reliably (done) only it's very fragile, upgrades to random packages broke it so often we now do our audio work without a general purpose computer in the hotpath.