My daughter took AP Bio through CTY this year (I realize that is not math, but it is an experience with a full CTY course). She found it interesting and provided a good depth of information. She learned more in that course than probably most other courses she has had in high school, but her high school is not a very challenging school to begin with. Knowing what she knows now after having taken the course, I think she would make the same decision again. The course was well structured and provided a great deal of information.
We have not had any experience with the Stanford Online Math courses for comparison.
Good luck!