Onto the details of his stuff ...
He's an extreme fly-ball guy. Some changeups are very good at getting ground balls; Hellickson's is not. It's slow and doesn't sink a whole lot, which leads to plenty of missed bats but not many ground balls. The change is certainly his best pitch, regardless, as he can throw it for strikes and pick up whiffs. Hellickson is very comfortable using the change against right-handed batters. The slurve has a nice swing-and-miss rate, but it's thrown for a ball quite a bit. His fastball doesn't look too impressive; it's probably best used to get over the plate early in the count to set up the changeup, but it is worse than average in zone rate and balls per pitch.
Hellickson's K/BB ratio is down to 1.7 after being consistently above 3 in the minor leagues. He's having trouble locating his fastball, and it's leading to way too many free passes (3.4 per 9 this year, with an unspectacular strikeout rate of 5.9). His changeup does look very good, though.
Here is a link to a 2010 article about Hellickson's repertoire.