I have used it before and had very good results. My brother uses HIT year round and has exploded like I have never seen. He is extremely strong and large.
Many people scoff at HIT and its principles, but most of its principles are well founded. The sad thing about most people who scoff at HIT, is that they are very misinformed about training. Many of them are used to reading the garbage magazines that promote ridiculously high sets and rep schemes. Mike Mentzer Heavy Duty HIT is based on 1 set to failure, which many people believe is ridiculous and for only the lazy. This is not true however. Working multiple sets to complete failure and beyond would constitute overtraining in most indiviudals.
I use MAX-OT, which is very much like HIT in many of its principles. You can read up on it at www.ast-ss.com, and its completely free. It explains everything in the program, and why each segment of the routine is done.
So, dont be afraid to try HIT. But then again, dont be afraid to try other things as well.