For us civilians, who would only be using it as a range gun, the standard AR gas system works just fine. A piston AR is nothing more than an expensive novelty that really isn't necessary for our use. (Has that extreme "Look Cool Factor" though doesn't it!?)
For guys facing real-word combat, under less than ideal conditions, and being forced to contend with extremely harsh climatic conditions that can incapacitate small arms without extensive care and maintenance, I can see where the HK would be attractive.
However, given HK's extremely elitist and anti-civilian corporate outlook, they can shove it up their arse. I'd much rather give my money to companies that appreciate all of their customers, no matter their profession, such as Swiss Arms, LMT, Armalite, STAG, etc...I still would love an HK MP-5, but after falling in love with my Swiss Arms CQB, I wouldn't trade the latter for a new-in-box MP-5, even if I had the opportunity.