Some folks and comp teams swear by it, I've never used it, and I wasn't that impressed with it. It does make for a moist piece of meat, however, I have seen streaks in meats where the person injecting wasn't as skilled as they needed to be, and it detracted from the turn in appearance of the box.
Just my 2/100ths
Treasurer, Great Lakes BBQAssociation
Rubbed, Smoked, and Sauced Competition BBQ Team-
22 1/2" WSM, 2 18 1/2" WSM's, 22 1/2" Weber Kettle