Chiming in to say that my 9th grader using Acellus and it gets more rigorous in the older grades. She is taking 9th grade classes plus geometry now and there are honors class options and AP classes at the high school level. 7th and 8th grades were not as challenging, but she was not functioning in public school by the end, so Acellus was a huge improvement. Also, she found that some of the math that she didn't quite understand in public school was presented in a clearer way through Acellus.
We are in FCPS so I can't speak to the MCPS aspect, but for the right kids, Acellus is a great program.