I will be following this topic because I still haven't found a good middle school option. AoPS is too expensive. It's clearly an excellent choice though.
I posted my criticism of Redbird on another thread. I said it was too low level and repetitive. The program has made improvements since then, enabling parents to skip ahead of the easy parts and the repetition. My son likes the videos. I like the long problems sets. Redbird doesn't allow parents to view the program componants, so it's frustrating when you are trying to use it as a supplement to fill in gaps. The vocabulary is not inline with the math, which I find annoying. It's a good CCSS test prep program. my daughter is using it to review math concepts since she's not using CCSS curriculum at school.
We have a subscription to IXL through school, but never use it. It's supposed to fill in that computation practice part that is missing from the new math programs. It doesn't have enough problems to master a concept though. I like the transparent table of contents. You can skip around IXL and move back and forth between grades.
My math genius friend uses Aleks with her elementary school children and likes it.
Reflex math is game-based, and I know it has helped struggling learners.
Time4learning is another program that has more games.
Khan Academy is good for mature children (not as playgroundy as the other commenter noted). I prefer Virtual Nerd.
