My DS attended LISG from 2005-2007, for 4th and 5th grade (he'd gone to a private school on the North Shore through 3rd grade and then we moved out west to attend the Davidson Academy for 6th grade forward). For the most part his experience at LISG was a positive one.
The nice thing about LISG is that your child gets to stay in his or her age-peer grade, while working at his/her own pace. In other words, all 11-year-olds are 5th graders, but other than homeroom the subject classes are ability-grouped. So, for example, some 5th graders will be taking 6th grade math, others 8th grade math, but all are still 5th graders. That was nice, especially coming from a school that had a tough time differentiating. The faculty is also excellent. My DS still uses his LISG English teacher, Mrs. Williams, as the yardstick with which to measure all other teachers (and they usually fall short). The teachers are accessible and understanding, but still expect quite a lot from students.
On the down side, the art and music program is a little lacking. In addition, since the school pulls kids from all over Nassau and Suffolk, friends tend to be spread out all over the place, which can make get-togethers difficult. The distance can also be a pain, especially in the winter. We lived in the Manhasset/Port Washington area and had to pay for private busing (which was often an hour-plus ride on the LIE/Seaford-Oyster Bay Expwy/Southern State).
The school is also fairly set in their ways, and sometimes can have a tough time thinking outside the proverbial box for students who need that. In addition, parents often felt kept at arms length. It was annoying, but not a dealbreaker. We might have stuck it out a little longer, but then one of his teachers was arrested on child pornography charges (not involving any students, thank goodness) and though it didn't really involve the school and they had no way of knowing what this man was doing in his spare time, I wasn't thrilled with how the administration handled the whole situation. While that was certainly a disturbing incident, and helped make the decision to leave a little easier, we left mostly b/c my DS was looking for more challenge and flexibility, which the school wasn't really able to provide. Still, DS enjoyed his time there and still keeps in touch with many of his former classmates.
While LISG, wasn't perfect for my 'wild-way beyond gifted' kid, I do think that, despite the drawbacks mentioned above, they do have the best program in the area. And for a private school -- especially one on Long Island -- the tuition truly is reasonable. But Grinity is spot on, you have to go visit and your child needs to spend a day in the classroom there in order for you to get a true sense of the place. When we applied, you had to submit IQ scores before arranging an interview/visit, but give them a call because that may have changed.
Good luck!
WOW