Though I've always thought <player idref= was one of the more underrated prospects in baseball and have made the extra effort to acquire him in dynasty leagues, I don't know that he's the type of prospect you should rush to add off the waiver wire in standard mixed leagues (like Anthony Rendon or Nick Franklin) even now that he's the starting shortstop for the Mariners.
He's not as good as his minor-league numbers would suggest, having benefited from some extremely hitter-friendly environments, but he's one of those scrappy types who tend to get the most out of their tools. Ultimately, I think he has the potential to be another Matt Carpenter (another overlooked prospect), but as he works his way up to that level, I expect his moderate power and speed to make him somewhat fringy.
I rank him 19th among shorstops with the potential to move up. If you've been plodding along with a Brandon Crawford or Zack Cozart type, you should take a flier on Miller's upside, but otherwise, you can wait and see.