I think the impenetrability was consider more of a feature than a bug by the person doing the recommending. IOW, he was assuming that people taking my recommendations would be geniuses by-and-large and that the harder book would be better in the long-run for the brightest people who studied from it.

You must have misremembered; Rosen's text is very verbose and clear. It will certainly be an elementary text for any second-year or higher math undergraduate.

The book is partly designed to be a crash course in math for CS students, so there are introductory chapters on proofs, logic, sets, etc., with plenty of examples. Exercises are mainly drills and computations, with a smaller proportion of exercises on proofs.

As for the bad Amazon reviews—many students using this textbook will be encountering mathematical proofs for the first time, so frustration from some of the students is to be expected. I don't think the reviews are representative of the book.

All in all, the text serves its purpose well as an introductory math book for CS undergraduates. I think its greatest downfall is its excessive verbosity—there is so much redundancy that examples take up around half the book.