There was an article on baseball america about it, and they said hard throwers at the top of the zone benefitted greatly becauase that's the strike that the human ump doesnt call. However, they also found the low strike that human umps tend to call, wasn't called.
They also found that the left and right side of the zone borderline calls greatly benefitted the hitter. No more human ump strike zone expansion on 3-0 and 2 strike counts, which is when the majority of those calls get blown by human umps.
Frank Viola is on one the teams in the league using it and he said the command/control guys really get stifled because they don't get the benefit of the human umps being swayed by framing etc. He also said, of course, that it's far from perfected and will only get better over time.