well, if I recall the threads here from a couple years back.. weight and the occasional annoying tendency to detonate the air-burst shell right as it left the muzzle.
Regardless,the thing was a boondoggle from start to finish.
As a stand-alone unit, the 20mm thing looked like it might have had some promise. Trying to make it hump a stockless G36 was just silly... the result looked like it had designed by kids with more time in Counterstrike than behind a real rifle. The XM8 that followed in turn looked like little more than a thinly veiled "save our jobs" campaign to salvage something useful from the program.
Personally, I'd say the results of the XM-29/XM-8 program and the SCAR trials epitomize the difference between gov't and private industry approaches to a problem.
Anyhow.. to the G36.. agreed I don't see us switching over. Not because of the "Not Invented Here" -- just no appreciable benefit for the cost.