I'm almost embarrassed to admit that I only recently started using a metronome. This lack of discipline is probably apparent in my playing, but I'm happy to have gotten over the hump of not using one only to discover that playing with a metronome can be a fairly gratifying experience.
Playing with a metronome is surprisingly calming and hypnotic. It allows you to identify trouble parts where you either speed up or slow down so that you can concentrate on smoothing those sections out. I was working on the reel Rakish Paddy the other night. After getting in sync, I probably played it over a dozen times through (with the clicker going), which is something I've never done before on my own.
Once you've identified errors and remedied them, you can practice playing the tune perfectly with the metronome to keep you on track. This will teach your neurons to play the tune in-time in a relaxed, steady fashion rather than sloppily and full of mistakes. By hearing yourself repeatedly play the tune correctly, the sound of the well-played tune gets ingrained in your mind's ear rather than a version filled with uncertainty and flubs. This breeds confidence and perpetual progress.