I have only recently adopted a metronome into my practice regime and dearie me I wish I'd done it years ago! I am self taught on guitar and have lots of sloppy habits that come from trying to nail things up to speed straight away.

Now that I am learning other stringed instruments (mountain dulcimer and 5 strng banjo)I finally decided to work as suggested by others on this thread. Starting at a slow bpm I can often get up to the desired speed for a particular ornamentation or break within a couple of weeks. And keeping a note of the bpm I have got up to with each practice is great for keeping track of progress and for being more systematic in my practice regime.