Reyes has a ton of skills, but he's just a 21-year-old kid playing in the big apple. His career so far has been hampered by nagging injuries and the gobs of media pressure that came with his progress through the Mets farm system. (See: Paul Wilson, Dizzy Izzy Isringhausen, and Pulsipher.) He's lightning quick in the field and has good bat speed. His eye at the plate is suspect and he swings at a lot of pitches, which leads to his low walk total. Although, he finally seems to be drawing walks occassionally now. But, also note that his K rate is quite a bit higher than it was last year...
People will continue to be high on him fantasy-wise because his talent ceiling is so high. But, until he learns to draw some walks and/or Willie starts running him more, he will underperform for fantasy owners.
My opinion: Wait until he gets through the majority of a season without going on the DL for an extended amount of time and expect him to go bananas the following season.
This year? Best to stay away or expect him to be an average fantasy SS.
So, yes, he's definitely over-valued in fantasy leagues at the moment. But in real baseball, there's a reason the Mets are so high on him and many other teams have tried to trade for him over the past few years.