I have the 2QZQ and use it coupled with my cuben tarp;especially in winter.In summer I might use it depending on where I will be camping and what the ground and weather conditions are expected to be.

I've found that in established campsites where the duff is worn down to the dirt the uqp shines when it comes to protecting the underquilt from splash back.In winter it blocks wind and helps hold the heat.
Mine weighs 5.6 oz in the stuff sack.