Mets acquired Santana for four of their top seven prospects. I personally think they gave up too much for one star and not get any prospects back, but either way, Santana is going to give the Braves headaches this year.
Mets have lots of great prospects in their farm system, so it shouldnt hurt them too bad. They really have been suffering from a lack of pitching and Santana really fills in that hole, a great pickup by the Mets.
The team that really screwed themselves here is the Twins. If they had accepted either of the original Red Sox offers or the Yankees offers back in December, they would have gotten much better prospects. They held out way too long. Carlos Gomez will be good, but I don't think Humber will ever be as good as once thought of and the other 2 prospects are too far off from the majors to know what will happen.
Of course, the Mets still have to sign him first, before the deal is final. Which, I think they will.