To answer your original question – yes, they really are that good.
I’ve just sat my first 8 exams & passed them all with an average of 95% (just in case that sounds like willy measuring, I honestly believe the results were down to BGS’s high standards rather than mine).
The course notes are excellent – in 8 subjects, I only managed to find 1 question that wasn’t covered by the notes, and of course you have the option of reading the manuals or using the CD-ROM. I found the computer-based progress tests particularly useful, as you get an instant appraisal of your progress.
As everyone says, the classroom phase is extremely intense, but the instructors have the ideal combination of knowing their subjects inside out, while being able to get across concepts in a way that even someone like me can understand. Believe it or not, we even managed to have a few laughs in general nav . . .
I can’t comment on other schools as I have no experience of them, but I can say - hand on heart - that BGS has served me (and a lot of others) extremely well.