Here's a video of Brian Manzella showing some drivers swings with an upward angle of attack, a level angle of attack and a downward angle of attack.
Here's still pics of the Manzella's swing along with some other data.
The advantage of hitting up is pretty clear here. Looking at his -3.1* vs. +3.4* swings he gained 37.2 yards in carry by hitting up (17.4% increase). Furthermore, his spin rate went DOWN by 299 rpm's (13.5% decrease). So he's likely increased his roll as well as his carry.
This is very similar to the difference in angle of attacks on the driver between JB Holmes (+3 AoA) and Tiger Woods (-3 AoA). Woods can hit his irons as long as just about anybody on the Tour because he has the clubhead speed and the angle of attack isn't nearly the same factor with the irons as it is with the driver when it comes to distance. But Holmes is clearly the longer driver of the two.
The debate came between whether or not the golfer should hit up with the driver. This leads to some debate. Some say that hitting up decreases the margin for error greatly. Some say that is not exactly the case and the LPGA is filled with accurate drivers of the ball that all swing up with the driver and optimize their distance off the tee.
I believe there's a tradeoff of accuracy vs. power the golfer has to take into consideration. I do believe that if a golfer were to hit down a little more with the driver, they would hit the ball more accurately with the downward hit, if anything because it decreases the impact of the wind on the curvature of the ball and if I hit a ball off line, the quicker I can get the ball to land on the ground usually leads to shot that is less hazardous.
However, I do believe that two different, but similar players can see one of them being much more accurate by hitting up with the driver over the golfer that hits down with the driver.
I thought blog follower John Graham brought up a great point about the upward hit. If you can pretty much hit it 35 yards further on the tee everytime, but miss two more fairways a round, what would you take? Of course, it depends on the severity of the miss of those fairways, but it's a great question in the debate.