Couple of disclaimers-
1) I have never been (and hope to never be) in a situation where my life depends on my ability to shoot someone.
2) I read grossman's book, and exchanged email with him, but it was quite a while ago.
with that said, I don't think much of his overall thesis.
First, SLA marshall's studies have, as others have mentioned, come under criticism...either Sheehan, in "Bright and Shining Lie" or hackworth in "about face" (sorry, don't have them handy to check) mention SLA Marshall coming out to the field, taking a couple of notes, and wandering off to write an update to his study. As others have noted, one thing he apparently did not control for was the degree to which individual soldiers could SEE the enemy- if fifty percent of the troops didn't fire, was it because they didn't want to, or because theyr were in defilade?
Secondly, some of his examples are bogus- the battle of rourke's drift, for example, where he describes brit soldiers holding their rifles out in front of them, as opposed to getting a good shoulder mount. he claims it's because the troops didn't want to hit...but the book "like lions they fought" specifically mentions that the troop's shoulders were beating to a pulp by extended firing of the martini .577 rifle they were issued.
Another bogus example is the kid who used a Ruger Mk II to kill several other kids at a prayer meeting. Grossman contrasts his hit ratio to that of veteran police officers, and concludes the kid was conditioned and trained to shoot that well. Bull. leaving aside whether video games condition kids to kill, walking up and shooting unarmed high school kid with a .22 is a lot different than exchanging fire with someone trying to kill you.
He may be on to something in a general way, and I certainly can't claim that I would be a super hero if the SHTF, but it seemed to me many of his examples didn't prove his thesis.