One of the problems, in MEL, is definitely (as goback said) the lack of reliability. I've gone up to the machines only to find them closed down, then having to go to the ordinary queue. This is really stupid on QF's behalf, as it further discouraged pax from using the machines.

When they are working, I find them excellent. Usually the queue is shorter than the J checkin one. I don't see any downside: If everything (seat assignments, etc) is ok, it's quicker and you don't have to waste an agent's time. If you have an issue with something, or the machine can't handle something, there's an agent to override/correct the booking anyhow. Even when I've needed human intervention, it's still been a quicker process than queuing up at the standard checkin desk.