Praise seems to be unanimous for him. I remember reading Kevin Goldstein saying he could be a #3 hitter on a championship caliber team, all of the BA guys being high on him, and Law thinks he could be a star.
The problem right now is that he's still barely played, despite being signed in '09. He just turned 18 a couple of days ago. Plus, the Twins are notoriously slow with their prospects, so if you want to invest into him, it might be awhile. Unless he's really, really, really good. Which he may be.
Here's what Baseball America's Ben Badler had to say in a RiverAveBlues interview back in '09:
"Sano will get a lot of money, and the latest rumblings from Latin America is that his bonus could be comparable to or possibly surpass the $4.25 million the A’s gave Michael Ynoa last year. With any 16-year-old player from Latin America, you’re going to get divergent opinions from scouts because of how young the players are and because of the context in which they have to be scouted. But I’ve had several international scouting directors tell me that Sano is one of the best prospects they have ever seen from Latin America. If the economy was better, he’d break Ynoa’s bonus record easily. There are teams that would sign him for $2 million, $2.5 million, but since the bidding is going to be much higher than that, they’ve just dropped out of the race and reallocated their resources to scouting other players. His body, his swing, his power, his athleticism and his arm are all highly advanced for his age, so he has monster upside. There are teams that have scouted Sano since he was 14 years old, but there’s risk involved with any 16-year-old prospect, especially one from Latin America."
July 2nd Signing Period Q&A with Ben Badler | River Avenue Blues
He ended up signing for a bit less than expected because age concerns became a question. They seem to be have been unfounded, though.