My 2p worth:
I used to play in a pub covers trio, and early on I bought a set of Trantec IEMs for myself and the singer/guitarist. We only put vocals plus a bit of guitar through the PA, so effectively we just used them for vocal monitoring. We always kept our backline. I left a few years ago but the same band is still using the IEM set.
Good points
- one small, lightweight case to carry around instead of 2 or more heavy, bulky wedge monitors
- very clear sound (after we bought decent earphones - the ones in the kit were shocking)
- isolation from rather loud guitar, bass, drums
- more space left on the floor, fewer obstacles for drunken punters to fall over
- keeps down the overall volume in the venue as ypu don't have another set of speakers blasting away on the floor
Bad points
- always recharging PP3 9V batteries to make sure they don't die near the end of the second set
- to start with you feel a bit cut off from the action (the downside of the acoustic isolation bit)
- my bass never sounded as good with the earphones in, as I was just getting whatever spilled into the vocal mics
- impossible to judge the sound balance - someone still has to walk out the front on a long lead and report back