I have a daughter in the Bellevue gifted program, currently in fourth grade. I think that the program is a good one, although of course it is not perfect. I agree that one of the greatest advantages is having her be with her peers. She went to a private school for kindergarten and first grade, and we switched her into the public schools when she qualified for gifted for second grade (that's when the program starts, although I have heard rumbles of a possibility of dropping the second grade program and starting in third). The private school (not a gifted school) was very expensive, and I really don't think we got our money's worth from it.
Right now, highly gifted elementary students in Bellevue are grouped at two locations, but I have heard that they are going to expand to three because of overcrowding at those locations.
I can't speak to the preschools - my kids were at a daycare all day while I worked. I have a son who is starting kindergarten at the same school that my daughter is at. I like his teacher and the program a lot. He is probably MG at best, though, while my daughter is a DYS.