The problem with HIT as I see it is that it can be a decent (at best IMO) system for a short period. The biggest problem with HIT, as madcow2 said to me, is the people who use it

They have a tendency to be EXTREMELY dogmatic - basically, proponents tend to claim that it's the only way to go, period, and that everything else is stupid/inefficient. Also, they tend to use analogy to back claims rather than science. That's a huge red flag because if you want to believe something, it's pretty easy to use an analogy to make it sound plausible.
As alluded to above, there are more factors in training than simply hitting failure constantly. Nothing particularly wonderful happens at the muscular level at failure, but there is significant nervous system activity that is generally thought to be counterproductive if overdone.
Extremely simplified generalization: few sets taken to failure and beyond = not enough muscular stimulation, too much nervous system fatigue. More sets done to short-of-failure: more muscular stimulation, lessened nervous system fatigue.
Not sure where you're at in terms of training experience, but here's a great thread with some debate on the topic. It's long but worth the read if you're up to speed on some of the terms. You can clearly see what I meant by characterizing HIT proponents as dogmatic (at one point, the HIT guy actually says that if he doesn't already know something, he doesn't need to know it

) :
http://www.fortifiediron.com/invision/index.php?showtopic=6685