I have previously purchased Prime headsets for my hand held radio also called a handy talky or HT for short.  I need the headset for two city RACES events a year.  Problem is that the Prime microphone fails to function properly after two events.  This failure happened twice in two years.  Buying these headsets every year was getting expensive.  I called my local Ham Radio Outlet store and explained my problem.  The salesman suggested I consider buying the Icom HS-51. It was expensive $80 but came with some neat features.  I could not find any Internet review.  I was desperate so I decided to buy it.  I have two HT units and I just love my Icom O2AT.  It is 2 meters only, really old, large, and heavy but has some really neat features that make it ideal for bicycle mobile work. The options for providing a headset for that HT were limited.   In short, I found the HS-51 to be impressive.  It was very well designed and constructed.  But I have three minor complaints.  The switch unit rotates easily from the belt clip; will not stay in place.  The Prime unit also rotates but has a positive lock function for set rotation points.  The belt clip on the HS-51 is rather small while the Prime is large.  The Prime has a much better mic switch.  The switch is a much larger button and is activated on the large flat surface of the plastic case and that switch is easily found when bicycling.  The Prime unit, I did not have to look for the switch but my hand could easily find it.  The HS-51 has a very small switch along the side of its case so one has to squeeze the case to be able to apply the necessary pressure.  The switch is also used for TOT mode.  Press the switch once and the transmitter is on.  Press the switch again and it goes off.  The unit will automatically turn off after 2 to 4 minutes if you fail to hit the switch a second time.  Now that you think the Prime might be a better headset, let me now list the over the top features of the HS-51.  It has superb audio.  There is an audio gain control.  The headset folds up.  The earphone piece is not fixed but is on a long enough wire.  The unit has TOT, PTT and VOX activation for the microphone.  The mic and headset connectors are split so it marries with more HT units.  Wearing the headset for 5 hours was almost as comfortable as the Prime.  We recommend this unit.

Glossary:

Headset = In this instance, the term headset is not only listening function but the unit also has a microphone that can be held quite close out in front of the person’s mouth for good close private audio quality.
PTT = Push to talk is a very safe mode.  The operator presses a switch that turns on the transmitter while providing audio to the transmitter but when the operator stops pressing the button, the transmitter goes off.
RACES = Radio Amateur Civil Emergency Service.
TOT= Time out timer is a mode where the transmitter is turned on but will turn off after a set time.
VOX= Voice operated transmit depends upon an electronic circuit that senses when the operator talkes and thus turns on the transmitter and provides the audio out to the transmitter.  VOX is neat in that the operator does not have to press a switch allowing for hands free operation.  The down side is if the circuitry can activate when a person makes a noise that you do not want to go out over the air.   We do not recommend VOX for parades where there is high noise levels that would trigger the VOX to turn on at the wrong times.