A desuperheater scavenges wasted heat from the compressor and provides heat for your domestic hot water. It is especially useful for air conditioning as this heat is removed from the home.
During heating a desuperheater will steal heat from your home. The the advantage is that your GSHP would have a COP of 3.5 or better. This means that this domestic hot water heat would be provided at 25% of the cost of resistive heat. That's the theory anyway. If it's not installed properly it may actually increase your electric bill. For heating I think it may be best to forgo the desuperheater and the extra hot water tank required. I think a desuper heater is required for the new 2006 GSHP tax credits though.