We use (Mic'd up and through FOH) guitar amps on stage so the guitar I'm hearing in my IEMs sounds like it's going through an amp.
IEMs do take a bit of getting used to - like someone above said, you hear all your mistakes with crystal clarity which can be an eye opener, and not in a good way.
Getting a good mix in your ears is crucial - we use Qu-You (?) so we all do our own mixes.
It does feel a bit weird at first, but once you get used to it it's fantastic, particularly if you sing - as much of your vocals as you want without them bothering anyone else.
I also gig in a non-IEM pub rock band (vocal PA, wedge monitor and ACG custom lug plugs), it's quicker and less faff to set up but the sound you're given is the sound you get.
I'd encourage everyone to try IEMs and stick with them for a bit, get a good mix and keep the volume at a decent level.
If you don't/can't/won't use them for any reason, use earplugs!
ACG customs are well worth the outlay - there's no amount of money that will cure tinnitus.