Several years ago, the metronome drove me nuts. My brain just couldn't integrate it with my playing, so I put it away and forgot about it for several years. But recently I got it out again, and it's helping. Who knows why?

Thoughts on speeding up and slowing down:

As someone said above, watch out for people who don't play long notes long enough.

People may speed up on easy passages, such as runs that go up or down the scale, thinking, "I can play this! Watch me go!"

People may slow down unconsciously for harder parts, such as those that have big jumps or unexpected accidentals.