WE have been there twice (me, wife, and our seven year old boy), and are going again this weekend...had a great time! out in the middle of nowhere, in a sense...but nice camping loops, great access to the water (no need to wallow through mud or climb down a steep rise or anything), and a nice 'overlook' of the lake at the old CCC building from the 30's...
have fun!
have been twice to goose island...both times, ended up on the 'beach' side rather than on the interior of the park...basically parking right on a beach, with no trees or anything....bad part about GISP is that there is no 'swimming' access...although right on the gulf, there is a sea wall and I understand swimming is verboten
have been into mustang island, but not really our cup of tea...just like at the 'beach' side of goose island, the sites are wide open such that you could, if so inclined and the neighbor's blinds were open, see what the neighbor was watching on tv from your unit...we usually prefer a bit more separation and privacy