According to Nick Lord (writing for The Mathematical Gazette), the two volumes "met with immediate acclaim from functional analysts as a clear, careful, self-contained introduction to C*- and von Neumann algebra theory", something many in the field felt was missing.
[1] Ringrose and Kadison wrote with a pedagogical goal in mind, purposefully keeping the references sparse and including a long list of exercises.
[2]: vii Gert K. Pederson wrote in 1994 of the volumes' popularity: The theory [of operator algebras] has certainly come of age and has become fashionable.
At the same time, the demand for concise textbooks that cover the subject has become noticeable [...] Fundamentals in the theory of operator algebras [...] have quickly established themselves as The Textbooks in Operator Algebra Theory.
[...] The 715 exercises serve to illustrate and extend the results and examples in the text, but also help the reader to develop working techniques and facility with the subject matter.