I've worked in a few bands that had certain parts sequenced so I had to play with a click. While I am comfortable playing with a click and have done so in the studio for many years - I found that playing live, I needed so much click in the phones that it really damaged my hearing in my left ear (the ear that took the feed).
The worst crash and burn using a click happend back in the mid 70's (mind you that was before MIDI and sequencers, etc.) The band supplemented harmonies and some horn parts useing a cassette tape that was hidden inside a dummy keyboard. The concept worked very well and rarely did people even realize some parts were fake.
However, one night at the start of a song called "Disco Infernal" which starts with a long, dynamic horn intro - the guitar player was so loud, I could not hear the click or even hear the horns (I had no monitor). I had to follow the guitar player (hoping he could hear the horns) - well he was rushing so bad that the whole song feel apart. We had to actually stop the song, re-wind the tape and try again.
This was in a very crowded club with a couple hundred people and at least 50 dancers on the dance floor looking very confused. After that - word got out that we were "fake" (again this was before sequencing was the norm) and it really hurt our bookings - the band folded shortly thereafter.