I just bought two of the 600w digital ballasts from HTG and, so far, they've been great. Previously, I used standard coil-based ballasts. These new digital models run almost completely cool and are much quieter than my old stuff. Also, checked their power consumption with a multi-meter and found that they pull about 15% less power than my old ballast of the same wattage.
FYI, I'm using EasyCool reflectors and the standard GrowBright HPS bulbs that come with the kit.