I have spent at least a few thousands of hours playing and practicing with a
metronome in my life as a guitar player. The benefits are nothing short of staggering and I would recommend it to any and all that play guitar and want to improve.
Music can be simply defined as
'notes in time' and that definition should compel me as a musician to take the
'time' part of the equation seriously. I remember a very famous respected studio musician saying that "
notes are secondary", and that, from a modern perspective gives
'time' a little more weight and importance than
'notes'.
Using a
metronome can be challenging at the start. To have a tempo or beat clicking in your ears as you play will take some time to get used to. Start off with simply hitting one note per metronome beat as you ascend through a simple pentatonic scale and then you can graduate to two, three or four notes per metronome beast in the future.
I use the metronome to practice scales, chord rhythm, song parts, and to develop speed and good phrasing in melodies and guitar runs. I usually practice with
slower speed settings so my playing remains flawless but some guitarist really push the tempo to play as fast as they can for the purposes of their style of music.
If you
buying a metronome, find one that has a pleasing sound that you will be able to listen to for hour without irritation or ear fatigue. I like small metronomes that can travel with me easily and that have the tone of a woodblock.
If you get to the place where you're playing music professionally, there is no way to avoid playing to some kind of metronome, so follow Vinnie's advice below.