Ability Grouping

I am currently working on my doctorate in Curriculum & Instruction and I will most likely complete my dissertation on class makeup, to include size, grouping, and inclusion. I am very interested in Jennie Oakes' take on grouping and the research to support it. I would also like input from classroom teachers.
My opinion of ability grouping is that if done properly, it is necessary at the lower levels. For instance, in Reading, students will gain more by reading at their instructional level, rather than another child's instructional level. This translates to small group lessons taught at appropraite ability level.
Comments? :-)