Aviom is great and they have options for using either a separate monitor mixer or using the individual monitor 'stations', allowing each user to dial in their specific mix based on what channels you feed into the audio interface.
But it's an expensive system. We're hoping to use this rig at my church in the future, but the cost (close to $10k for everything we need) is putting off us buying it for a while.
Roland/Boss has a new in-ear personal monitor system available now and I have a friend who uses it at his church and likes it, but I know nothing about it.
Another option is the Hear Back system. I've used this system at a friend's studio and it works well, but it has its quirks and limitations. It is more cost effective, though, and might be worth looking into.
There are cheaper alternatives...using random headphone amps and routing a general stereo or dual mono mix from the board to the headphone amp or using the aux outs to feed headphone amps, but you're extremely limited on individual mixes (everybody gets the same mix or less-variable personal mix) and may not have any volume limiting capabilities (built into the other systems in order to protect people's hearing).
You could do this if everyone agrees to having the same/similar mix AND agrees to having mix right/click left...which I would personally find very annoying, but that's me. I typically only wear one side of a headphone when singing/playing because I hear and perform better that way.
I'm not saying that the mix one side/click the other is not a good idea, but everyone may not be on-board with it.
Which reminds me...
One thing to consider when using ANY personal in-ear monitor system is the use of an ambient room mic and having that signal put into the monitor mix. The major complaint with in-ears is the total isolation of the mix from the room and that mix being too dry or cold or close, etc...
The way to overcome this is to have a microphone (or a pair) placed on the stage in order to pickup the room sound. This mic gets fed to the board and the signal ONLY goes to the in-ear monitor system, allowing those with the in-ears to get a mix closer to the environment that they're in and less dry and unattached.
The point on having the mic on the stage is to remove any delay that would be caused from having located away from the performers and their ears, thereby throwing off people's timing.
