Baseball America — who list him as "Amed Rosario" — ranked him eleventh on their list of the top international free agents, and had this to say about him in their scouting report (subscription required):
Rosario might be the most divisive player in Latin America. He has a long, lanky build, good bat speed and raw power in batting practice along with average speed. Some scouts who like Rosario enough to have him ranked as the top prospect in the Dominican Republic, seeing him as a true shortstop who with power who can hit in games. He showed that at the MLB showcase in February in games against Venezuela, going 4-for-7 with a double, a walk and no strikeouts. Supporters like his fielding instincts, hands, arm strength and ability to make the barehanded play.
Other scouts see an upright hitter with a leg kick that gets him out on his front foot against offspeed stuff and leads to strikeouts with his uppercut stroke. His body has a lot of room to fill out, so he may end up at third base, but some scouts aren’t sold on his infield actions and see him as a corner outfielder.
For what it's worth, Baseball America also mentions that scouts rave universally about Rosario's makeup, and while we can debate makeup vs talent until the cows come home, with all other things being equal you'd always prefer the player who comports himself in a manner befitting his talents and exposure.
As Adam Rubin points out, the $1.7 million is the biggest bonus the Mets have ever awarded to an international free agent, $400k more than they gave Fernando Martinez in 2005.
Here's some video of Rosario playing shortstop and swinging the bat: