The material needs a careful proofreading by an outside reader. Besides one or 2 typos (usually minor) per page there are some careless mathematical misstatements that can be very confusing for students. (These were in the ``Field Guide to Functions'' section which may have been written separately).
Every ``Stage'' could do with more exercises, including more elementary exercises. The jump from the material presented to the understanding required to do the exercises sometimes seemed quite large, especially for an independent reader. (Of course, it must be remembered that the site is designed to be used for an online course, and registered students can discuss problems with the instructor or their classmates.)
The material is presented largely from a `reform' viewpoint, as in Stewart's Calculus: Early Transcendentals, a book which is in fact recommended as a supplementary text. The rigorous definition of limit, and rigorous proofs of limit theorems, are present but definitely not emphasized. Depending on one's personal preference, this may be either a point of concern, or a feature of excellence.