I did HIT, and it seemed to be the best thing ever if you love being sore for 6 days.
As for switching exercises, Mentzer never suggested this- he wanted you to stay on the program because of specific reasonings. everything was as efficient as possible in his eyes.
Personally, I'm a larger 225lbs than I was at 235lbs when doing HIT. my arm measurements are actually larger, and my legs are easily the same size. I'm a powerlifter now, using the Westside Methods. I guess you need to look at your goals, look at the top guys and figure out what's the best program to use.
I will warn you that Mentzer's program basically ignores an adequate volume for the upper back/rear delts. I HIGHLY suggest supplementing the program with more rear delt work, and perhaps using Rows instead of pullovers or pulldowns (substitute one for Rows).
PM me if you have any real questions, but I'd recommend you don't stick with HIT. look at what he's saying (focus on recovery, proper technique, proper form) and apply it to your current routine. perhaps you will notice changes without spending a lot of (what i consider) wasted time on HIT. you're not gonna get strong, and you're sure as hell gonna spend a lot of time sore. Be prepared.