The school district I work for uses 10.5 on all its computers. However, when we upgraded our servers to 10.5.8, that created a bunch of problems. Our tech guys asked Apple about it, & they said they're known issues, but won't fix them. Which is awful. The reason we don't upgrade to Snow Leopard is
a) still a little buggy and
b) We like to wait until the following summer just so we get used to it. Plus, having do a major upgrade like that during the school year will cause many service interruptions. Plus, consider we have 1000+ computers in the district.