Just to add my slant on what's been said ; I have used IEM's for nearly two years now and I am absolutely hooked . Our drummer does the sound for our band and got us all converted. He has used two IEM's for years and has a totally confined mix of his own, whereas the other three of us use only one IEM and monitor the live sound of the band with the other ear.
I go from my bass(es) through a DI box straight into our Yamaha O1V desk. As I am the main singer, I have my vocal [i]and[/i] bass in my ear, and the other members vocals set back in my own mix - we do up to four part harmony so it's important we can all hear each other. I pick up the kit and two guitars ambiently with my free ear, and the rest of the band get my bass and vox through their own IEM at a level they want. And without the need for other monitors we always have lots of space on stage.
It works brilliantly, the sound I hear is perfect and the overall volume is adjustable by me. Admittedly it takes a bit of getting used to, not having the thump behind you from an amp but no longer having an amp to lug around any more is more than worth it..
I can hear the purists wailing but when you're getting close to fifty, it doesn't half make a difference every weekend
I can thoroughly recommend anyone who may be wavering to give the IEM route a try - I know it won't suit everyone, but I think it may well be the future for a lot of us
So, technically speaking, if anyone asks what gear I use, my rig is our JBL VRX line array PA system!