The same was true for YouTube comments, and that's why they switched to Like/Dislike buttons. When it comes to judging the value of things, people tend to be all or nothing.
That's more the case if you show some sort of aggregate 5 star rating on the front page, I suspect. If you look at peoples iTunes libraries they probably use 2-4 stars a lot and 1 or 5 stars more rarely. But on Amazon, people who think a book is 4 stars but see it rated as 3 stars are going to vote for 5 stars to push it up more.
The first one is by Philip Wadler, who's quite famous in Haskell circles. The second one is by Matthias Felleisen, is quite active in using Scheme to teach computer science.
Nobody of those says that SICP isn't a really great book.
Wadler claims that the book and a 'better' language would be an improvement.
Felleisen et.al. claim that the book does not prepare the students for software engineers in industry, is very difficult and thus is not the best book for a typical introductory course.
Even though I understand both critiques, I see their point of view and find both views valid, SICP is still * out of five stars.
Even for those who love SICP (and I count myself among them), reading TAoCP cover to cover would not necessarily be enjoyable or fruitful. It's useful as a reference, but not as a teaching text. (IMO.)
Not true now [1] and not true back then [2].
[1] http://portal.acm.org/citation.cfm?id=1134743 [2] http://econpapers.repec.org/paper/eclstabus/1670r.htm (Fig. 1)