I think it varies hugely from small school to small school. I've known a couple that were brilliant, and at least one that was dire. A lot will depend on the staff - if there are only 2 or 3 teachers then you need them all to be fabulous.
DD went to one - about 39 pupils when she started and more like 30 whenshe left. IT was fantastic. SHe got a real sense of community, lots of personal attention and a collection of friends for life. The mixed classes were not a problem academically and had some real advantages. However, as a teacher I think they must be a nightmare to manage!