After dragging one around Iraq for 14 months, I can say that while it is a lighter, more handy weapon than some, The M-16/M-4 has a serious flaw...it's direct impinging gas-blowback action.
This design is light and simple but can and does lead to failures due to fouling.
It sprays carbon and un-burned powder residue back into the chamber and bolt-face area with even just a little firing, and it is extremely hard to clean, especially the locking lugs on the breech face.
The HK-416 is basically the same M-16/M-4 design with the addition of a gas-piston and operating rod (like an M-14 or M-1A).
Because it is not spraying gas and residue back into the action, this weapon stays cooler during sustained firing, fouls less, and can be fired much longer before it needs a cleaning. This makes it much more reliable.
Also, HK's proprietary (smoothly curved) steel-bodied magazine design is clearly superior. It is a little bit heavier than the standard issue 30rd M-16 magazine, but it works reliably when fully loaded, suffers less from bulging, it is harder to damage the feed lips, and easier to clean.
If I could find one, this is the AR-15 type weapon that I would choose to own.
However very few have made it into the civilian channel, as Colt has chosen to file lawsuits against H&K, claiming that they are infringing upon their patent and manufacturing rights.
CptA