i had one on my 03 z71, it wasnt bad, but doesnt even compare to the performance a custom tune can put out. i noticed with it on the reg fuel setting it did little to nothing, with premium it did make a difference, main reason i bought it was to change for tire sizes.
if its a killer deal for like $100 or less then it might be worth it and if u want more then just sell it on ebay or something urself for 100-150.
the biggest thing custom tunes have over the handhelds is the ability to remove torque management, it makes the biggest difference.
bully dog does sell a handheld that ive been told removed tq management but is more money than most handhelds and closer to the price of a custom tune.