Varies by library. In 2000 we didn't have any service calls. I don't think we've had any this year. As long as their first experience with the machine is a good one they will use it again. Again, depends on the library.
None go long stretches idle.
At some of our busier branches, without self check, waits can be longer during high volume times. All are near the circ desk. The closer the better. Right on the circ desk works well. Quick, easy and private.