I've read an awful lot about him before this year and especially this year. A lot of the hype has been justified and some not.
Fastball - some starts it sat around 90-91 and peaked at 93. He didn't do so well then. But many of his starts he was around 94-96 consistently. When he was throwing 95 he also threw a 2 seamer I believe. That or a type of cutter. His location was fairly good but the usual young players lack of command (mistakes in the zone, etc.).
Slider - This was the pitch many raved about. Even guys using pitch f/x saying it was the best slider in baseball. I guess they need to turn a tv on and watch Zack. Or just turn a tv on and watch Brett try and locate his for a strike. It's an excellent out pitch and he does locate it once in a while. But it's definitely not a plus plus pitch. To me it's borderline plus. Borderline because when a hitter swings they won't be hitting it, but it can't be located well enough.
Change - This is the pitch I love. Nice tight late break. Very repeatable. Good location in and out of the zone. Nice arm action. I've seen a 2-0 count followed by 3 changes for a strikeout. Sweet. Needs to use it a lot more against lefties, especially on days where he doesn't have his best fastball.
I have a little concern for the large velocity spike. For some reason guys with a big jump tend to get injured the next year. I'm a little less concerned after hearing about the workout regimen contributing. And I hope that his velocity is more consistent throughout the season.
But even without the velocity I think he could be a good pitcher. He would just have to use the change in more 0-0 counts and mix his pitches better. That and locate his slider better in the zone. Because when he is throwing 90-91 even with movement it seems quite hittable, maybe it's just very visible from the hitters eyes (not hidden at all).
With his velocity though he could be an ace this year. Maybe not a Greinke/Felix/Halladay type yet. I expect he will have a few days where it will come apart. Having that focus all year and limiting damage is a big progression. But I think he has the stuff to become a true ace. Sooner rather than later. Be excited.