I know it's not an IEM discussion but I think this is relevant. The biggest drawback I've seen with our 5 year IEM experiment is with the drummer backing off on his playing because he has the mix loud in his IEM monitors. This also happens on vocals. He backs off because he can hear himself so well. It takes some getting used to that's for sure. Getting the monitors right during sound check is still pretty critical even though he has easy on the fly adjustment with the Ipad.