I did HIT for ~1year, actually might have been even longer. I noticed some great gains intially - body weight and strength kept going up, just as fast as any of my buddies who did 4X, 5X /week (mine was only 2x/week) - and it worked great for months, it was quite surprising i was making gains when working out SO little - we're talking ONE set per body part per week !!!! i wouldnt have believed it if it hadnt worked
but then i stopped gaining - this might have been due to the fact that training only 20mins 2X/week just started having a mental effect on me, training was turning into less and less of a lifestyle, so my eating got sloppy, etc... i found it really hard to keep the calories up when training that infrequently
i'm back to trying a higher volume approach for the next several weeks, and i've been making pretty great gains, i may do HIT in the future during down times again