I wouldn't say Shack is over rated... but you can definitely find great waves in the San Juan area, with the convenience of many restaurants, hotels and clubs steps away from the kiting spots. If staying for a week or more, I would suggest staying at San Juan. You can take an easy drive to Shacks at Isabela any day - its less than 2 hours.
The great thing about the kitesurfing spot ins San Juan, specifically Puntas Las Mariás and some others closer to Ocean Park, is that the waves are in outer reefs where you will rarely find paddle surfers...just kite surfers. SUP will be there when theres no wind.
About Isabela, theres a spot east of Shacks which I prefer. The spot is called Montones. Its a small bay, with villages right at the spot (Villa del Mar Hau)
Shacks is an awesome spot, visited frequently by the guys from REAL and many others. The thing I don't like about Shacks is that the shore is mostly rocky and you gotta watch out for sea urchins in those rocks. Also - don't plan on staying for a month at Shacks.
Theres no doubt that any of these spots @ Puerto Rico will be nicer in terms of wind and waves than Florida.
videos of Shacks:http://www.youtube.com/watch?v=izeZ1KVn ... re=related
Puntas las Marias (San Juan)