I have an Eponaire one way system. The earpiece isn't great for the rider - I prefer the more expensive systems (Comtek - also sold by Eponaire) where riders can use their own headsets. And the volume control has to be taped or it is too easy to move.
But for an inexpensive system is does all that is needed and once riders are used to the earpiece it is easy enough and stays in well. I can also hook the receiver up to speakers which can be useful!
A good "starter" system is going to run you a few hundred bucks, and the Comtek systems are over $1000. But well worth it!!!
ETA - my event students also have the VOX two way system for the X-country course, which is also nice, but as is stated on the Eponaire webiste, not great for arena work as the VOX is obnoxious!!
They say money doesn't buy happiness -- well happiness doesn't buy horses!