My son went there from K5 to 7th grade.
Some things I noticed:
The kids behave. It is tough to get in, so the parents ensure their kids behave.
The older kids look out for the younger kids, which is fostered by the school. Older kids will read to/with the younger kids, for example. Older kids remember when they were younger, so treat them more like little brothers and sisters, than someone to pick on.
It is a research school. The student body is a reflection of the state's demographics. So, the school isn't predominantly one race or rich or poor. I think that, combined with being with older/younger kids and being able to go through school from K5 through high school graduation makes for a unique situation that allows them to be better adjusted to the world around them post high school.
The facilities are nice. They incorporate technology into the classroom. They try different learning techniques/programs.
My son never complained about other students or any of his teachers. Before he started they had a principal who, from what I heard from teacher friends, wasn't a good guy, but he is long gone.
It is a smaller school, but I think that is to its advantage. If I has stayed in Tallahassee, I would have preferred FSUS to any of the other schools in Tallahassee for the reasons listed above.
It may not be a perfect school, but I don't think there are any, so I was pretty happy with what FSUS had to offer. Hope that helps.