I'm not such a big Metronome user. It's quite fun, though, as you never know what attack you might wind up with when you use this move. Honestly, I'll go as far to say that nothing could ever be as exciting as this. However, only time you'd see me using it is when battling against friends to pass time, and not for anything serious (such as battling competitively). I've also used it in-game a couple of times throughout story-line; last time I've done this was with Pokémon Diamond, which was about five years ago or so. Before that, I believe it was Emerald.
Wort experience I've ever had with this attack is when I had Clefable in my team at Battle Factory in Pokémon Emerald. It was my last member (other two were fainted completely), and it didn't have any offensive hits aside from Metronome. When I'd chosen it, though, it turned out to be Explosion (or was it Self-destruct?), making it instantly faint. It was pretty frustrating, considering how far I've gotten into this facility at the time.
As for best, one of them has probably gotta be when my Jigglypuff wound up with using Sacred Fire. It was pretty amusing to see!