My children were in multigrade classrooms in these grades, and up to third grade. They liked it a lot, and so did my wife & I.
One advantage is longevity with the teacher who has a longer period of time to learn how to teach to your child, and it gives you the opportunity to establish a better relationship with the teacher. Another advantage is that you child has a a chance to associate with kids that aren't exactly the same age and she'll learn to get along with a larger variety of kids.
I really can't come up with any strong disadvantages, but if I had to make some up, they would be that it might be harder on her later when she does change teachers ever year (but we get used to that quickly in middle school) and possibly that she might not get as much attention on the subject matter at her age, since the teacher has to divide the curriculum up across grades. But I think both of these are a stretch.