Rambam's position is rejected by all other Rishonim.  The position of the other Rishonim is pretty well summarized by the Sefer HaChinuch [which you can read here about half way down][1].

Regarding the Chatam Sofer, he says:

> אי אפשר לעשות להרמב"ם אפילו סניף בעלמא כי שיטתו דחויה, ולא מצאנו ידינו ורגלינו ברוב הסוגיות

So yes, Chatam Sofer rejected Rambam's position.

This is in his responsa, Yoreh Deah, קמג

However, many modern Rabanim are stringent when dealing with issues such as early immersion due to IVF and similar issues, because they feel Rambam's position needs to be considered, and that basically no one can ever know for sure if they are niddah or zavah under Rambam's method. One example I'm aware of is Rav Binyamin Lau who I've read is stringent due to Rambam's position.


  [1]: http://judaism.stackexchange.com/questions/31245/niddah-and-counting-7-clean-days-what-are-its-origins/47814#47814