CBS recently published a story criticizing 5.56 NATO ammunition used in the M16 rifle, M4 carbine, and M249 Squad Automatic Machinegun. Tamara has an excellent rebuttal of their rhetoric. She covers all the bases well.
I'm not incredibly enthused by the M855 ball generally issued to US troops. It was designed for penetration, but I believe it sacrifices terminal effects in order to achieve this. I prefer either the lighter M193 ball used in Vietnam or the heavier 69 grain and 79 grain bullets coming into use with the special forces. Those tumble better and generally have superior wounding capabilities.
But the 5.56mm concept is a good one. It provides much more ammunition capacity for a given combat load and I believe it to have sufficient lethality on the modern battlefield. 7.62mm and larger rounds are not death rays and many of the arguments used against 5.56 are specious at best. I just think the current military implementation of 5.56 is not the best one ever. But that is what we get for copying munitions designs from francophones.