I've never had it done to one of mine, but I understand the rebuild is pretty comprehensive. What you get back is essentially a brand new machine. They go through and rebuild the motor, replace the outer bag and all the worn plastics (if they no longer stock plastics in your model's original color, you'll get generic gray/black plastics instead), and polish up the aluminum to a scratch-free, mirror shine. All that said, I doubt they send you your old parts back. If there are things you'd want to keep, better remove them before sending it in.