I have the 3/4th inch Heavy duty (Item #66984) and 1/2ths Air Impact (Item #95310) both work great. I use the 3/4ths on my budd lug nuts and anything that needs 1000ft lbs of torque.
The 1/2in works great on my regular cars and its helped me rebuild two motors.
I do have a 3/8ths impact, but its a craftsman and its pretty useless. Spend the $$ and get the 1/2 in from HF.
Most of the reviews on HF are probably air supply issues. The 1/2 in needs 5CFM @ 90psi. Most people don't have a compressor that will run air tools, or try to run 100ft of air hose. You do know that if you have 50ft of hose you will lose a substantial amount of PSI due to length and the small diameter. All of my hoses are 3/8s, and i have air hookups every 10 ft. I do have 1 50ft hose, but most are 25ft.