Kevin Kiermaier set an all-time mark of sorts in 2015: The reigning Esurance MLB Best Defensive Player had the top statistical defensive season in recorded history, saving 42 runs with his glove, per Defensive Runs Saved. Obviously, that comes with a pair of enormous caveats. The first is that "recorded history" for DRS goes back only to 2002; the second being that it was his first full season, and we know that the current defensive stats generally require more than a single season to be reliable.
It's a question that vexed me as I put together a list of Top 10 center fielders for a statistically oriented segment on MLB Network's ongoing positional ranking series (Thursday nights, 9 ET). How much stock do you put into that DRS mark? Is it enough to overcome Kiermaier's uncertain offensive profile, particularly a .298 OBP?
Ultimately I slotted Kiermaier at No. 6, behind A.J. Pollock and just ahead of Adam Jones and Carlos Gomez, but it felt like avoiding the issue. If you fully buy into the glowing defensive numbers, then his elite glove made him a 5 WAR superstar as valuable overall as Chris Davis or Anthony Rizzo. If you don't, you cringe when you see Kiermaier is the only player with a below-average hitting line included in the top 30 Wins Above Replacement rankings.
Because of the uncertainty around defensive metrics, we still don't quite know the answer to that question, but that's one of the things that makes the emergence of Statcast™'s radar tracking capabilities so appealing. Rather than rely on some of the human judgment that goes into DRS (and a similar metric, UZR), the new technology allows us to measure every moment and movement of a defensive play, hopefully allowing us to reconcile issues with positioning and shifts to improve the performance of our fielding stats.
We're not quite there yet, but the wheels are in motion. For example, if we really want to see what kind of range Kiermaier had, we can simply overlay his successful catches with those of 2015's lowest-rated center fielder, Angel Pagan:  

The difference is stark, as Kiermaier (shown in the darker color) finds his way much farther around the center-field area to collect outs. This isn't meant to pick on Pagan, who was once a quite capable fielder but turned 34 and played through a season's worth of knee woes in 2015, so much as it is to highlight just how many more balls Kiermaier tracked down. It's not hard to see where one outfielder saved his pitching staff a ton of outs that the other did not.
But we know Pagan didn't have his best season. What about another American League East center fielder known for his defense, Toronto's Kevin Pillar, who tied for third in DRS and made more than a few highlight-reel grabs of his own?

It's not quite as big a gap, though Kiermaier still clearly covered more ground. The images line up well with the tracked numbers, which show that Kiermaier's average distance covered of 58.5 feet on catches was well more than Pagan's 51.2 or the average center fielder's 53.8 feet. 
Now, let's be clear about what those images are showing and what they aren't. Those dots are all catches, without concern for hang time or launch angle, and they don't show missed balls. Ultimately, there's an argument that since (in theory, anyway) many types of balls should be caught by literally every capable Major League outfielder, all that's relevant is showing the extremes, the ones that a great fielder like Kiermaier can get to that lesser defenders cannot, and eliminating the rest. We need to show averages, and perhaps ranges of success. This is a very good first step, but just one of many.
Still, you can see where this is headed, as our director of baseball research and development, Daren Willman, later expanded on in a tweet that included Kiermaier's starting position for each of those catches (in red):

Back to the original question: Should you buy into the defensive metrics on Kiermaier? Clearly, the Rays believe in his talents. As a prospect, he was named the Best Defensive Player in Tampa Bay's organization in both 2012 and '13, and he was added to the Wild Card roster as a defensive replacement in 2013 before he'd even made his regular-season Major League debut. Kiermaier was one of the few outfielders to top 100 mph on a throw this year. 
The defensive metrics believe in Kiermaier. From our initial looks, so does Statcast™. It's not likely that's going to change no matter how deep into the data we go; after all, you've seen the incredible plays. What will be truly interesting is hopefully determining if it's true that Kiermaier is really that much better than everyone, as our current decent-but-imperfect metrics say. That's the next step. We're working to get there.