I had a friend that took their online Mixing and Mastering with ProTools class. From the little I saw, I was impressed. I would say that 90% of the lessons were online (text combined with really good Flash illustrated examples) and 10% relied on a book.
Here's the good and bad part: the course was 10 or 12 weeks. There were alot of assignments. Even though he had DAW experience, some of the mixing tasks took him a very long time - especially because he was a bit of a perfectionist.
Anyway... the class began with about 15 students, by week six or seven there were only like three students remaining!!! And I think it cost around 1k for the class. By week six my friend spent several after work evenings and the entire weekend working on that week's assignments. He bailed out the next week.
It seems to me that if you can completely devote yourself, it can be very beneficial. But if you can't, you might just be throwing your money away - besides, isn't there a "Instant Film Score" plug-in somewhere that costs less than this class ?