Again, receivers are required for all users. The receivers and transmitters must be in direct line of sight of each other. This reduces the amount of flexibility you have in movement within the room without interrupting the signal. Unlike FM, you cannot cover the receiver or put it anywhere where the direct line of sight will be blocked (like clipping it to your belt and sitting in a class).
Quality varies with the company, here, too. Only purchase systems from dealers with trial periods and return policies, and who offer troubleshooting on the phone or in person.
Large areas require multiple emitter panels, which will increase the cost of the system.