Desuperheater is a useful feature to provide heating for DHW tank, however when I carefully read various document, please correct me if I'm wrong.

(a) Desuperheater only work when GSHP is active, it is not on-demand basis
(b) So the climate need to optimised to ensure GSHP is operating all time.
(c) It has different performance for heating and cooling.
(d) On demands is better using valve switching between DHW and central heating/cooling.

For climate where 70% heating and 30% cooling operation take place, I think desuperheater does not suit.

Can anyone link me to useful document or papers detailing how desuperheater performs and recommends alternative solution

It is curious that not all EU heat pump has desuperheater and USA/Canada/China do......

A. That is correct, a Desuperheat only works when the system is running.
B. You do NOT want the GSHP to run all the time, the desuperheater is only preheating the water so the hot water heater doesn't have to bring cold water all the way up to the hot water you expect when you take a shower, also it's helping to keep the hot water already in the tank hot until needed. You still require a Hot water heater that is either electric or Gas, a Desuperheater alone will not heat your water hot enough. Since your not running the hot water all the time, and the hot water heater isn't running all the time to keep water hot, the GSHP doesn't need to be on all the time. 
C. A desuperheater is most efficent when cooling your home, In cooling mode, it's extracting heat from your house and using some of that heat to pre-heat your hot water. In Heating mode, it's basicially robbing the heat pump of some of the heat you need to heat your house, but even with that loss, it's far more efficent to heat your water with a desuperheater than to rely only on the electric water heater element (or Gas). 
D. I would say Point of Service On demand water heater are more effiecent. But only POS units, that is a unit for you kitchen sink, a unit for the shower, a unit for the dishwasher, having a unit for every point that requires hot water can be a very costly initial install. 

I disagree, a Desuperheater is ideal for any climate. 
  

Refrigeration process has hot gas discharge. So wether you are heating or cooling with a GSHP something is available. Mind you on demand DMH is not popular here.
You have given me the perfect place to reveal recent results from a customer who's (independantly metered) electric water heater rates went up with an un buffered tank and a DSH.
"The adjustments you made on to the hot water right before Christmas (included new buffer tank) really did the trick. The kwh usage for the hot water was down 29% for December and a whopping 50% for Jan......"
We're in MI (definately heating dominated).
J

Desuperheater can provide anywhere from zero to 100% of domestic hot water virtually free during cooling season and quite cheaply during heating season.

Yes, it only runs when system is heating / cooling house, but most climates require quite a bit of one or both.

desuper piped into a buffer tank that then feeds tankless on demand water heater is a viable option able to provide significant savings.

Hot water is typical household's 2nd most expensive energy user behind space heating / cooling, so potential savings are significant

I live in Minnesota where we're probably in a 80/20 heat-to-cooling environment. My system just went on-line last week. I had no hesitation in ordering it with the desuperheater option. Our annual DHW costs are around $800. Even a 50% cost reduction (to me) would be significant.

I also feel that DSH will work in most climates. I am in the northeast U.S. and have had my geo system installed since last September. I have installed submeters to read electricity usage on my geo system, my aux + blower system and my electric hot water heater. Here is a summary of electricity usage results.

In the 98 days between 10/21/08 and 1/27/09 my electric hot water heater typically pulled 1.75 KW per day. There were outlier days (6-8 days) where it pulled more than that due to sporatic circumstances (after power outage, fall cleaning of all my curtains, etc).

Because of these few (but high) outliers, the average daily use is brought up to 3.37 (a total of 329.79 KWh was used during the 98 days).

So, for over 25% of the year, I've only used 329.79 KWh which translates to less than $40 here in upstate NY (@ 12 cents per KWh). Considering typical yearly costs to be $600 - $800 range, I believe that I am going to do well this year.

But, keep me honest and check with me this October when I have a full year worth of data!

I have my electric use as a trend chart here:
http://www.geochoices.com/results.html

I'm in Texas and my DSH puts out 140 degree water during the summer.  Down here, we're mostly concerned with oversaturating the ground with heat so I am working with my contractor to adjust the desuperheater so that it heats water in the winter as well.  I want to to remove as much heat as I can during the mild winters we have.

Our dsh was just hooked up last week, after the geothermal was operational for two months; backordered pump or something.... Anyhow, since the dsh was hooked up (to a buffer tank) we have been able to turn the setting of our on demand electric Steibel down 8 degrees to get our hot water, and gallons per minute has increased...

Perfect so your system was not completely operational (in service) 'til '09 :)