In the book "Black Hawk Down" one of the Delta operators talked about how they were having problems with the ammo they'd been issued and being able to stop the Somalis in their tracks. The Army had issued them carbide tipped rounds which were great for punching through light body armor and such, but would just pass through the unarmored Somalis and would not kill unless they were struck in the head or heart. Granted, this wasn't every person they shot, but it was enough that some of the Delta guys were frustrated. A mention was also made of Sgt. Randall Shugart, one of the Delta snipers who went in to support the Super Six Four crash site (and went on to win the Medal of Honor with MSgt. Gary Gordon). The operator told of how Shugart stuck with an accurized M14 sniper rifle, even though it was a fairly large and ungainly weapon compared to the M4s some of the other operators carried. His defense was that he preferred whatever he shot to stay down (referring to the 7.62 round).
My own personal feelings are that 5.56 is a very viable round, and it's been doing rather well for a few decades now. It does have some advantages over larger rounds, including lighter weight (you can carry more of it) and there are some extremely good platforms for the round that are versatile enough to both engage at a distance and be compact enough for room clearing operations. The History Channel just did an extremely good Modern Marvels episode on the M-16 that was very informative, if you get the chance I highly recommend it.