My main complaint about The World's Religions is that it is rather superficial in places. Smart has a list of the different dimensions of religion (doctrine, experience, ritual, myth, etc.) which he sometimes uses as a crutch even when they are not really applicable to the particular religion at hand. Also, while a reasonable amount of coverage is given to African religion, no particular religion is looked at in any depth, giving it an appearance of superficiality (the bibliography lists several general books on African religion but no ethnographic monographs). Smart also has a tendency to view religion as a thing-in-itself, without considering the complicated relationship between religion and other social systems, or between religious belief and psychology. No attempt is made to tackle the big questions about religions — why they exist and how they persist.
A few minor things also annoyed me. We get the usual inane attempt to turn "humanism" into something similar to a religion (that the two people cited as humanist "leaders" are as far apart as Bertrand Russell and Jean-Paul Sartre gives you some idea how idiotic this is). And for some bizarre reason (keeping on good terms with the creationists?) he insists on putting capitals on the word "evolution" (except when talking about Bergson's "evolutionary philosophy", where it would have seemed quite reasonable).
In conclusion I wouldn't recommend The World's Religions as a reference work — a good encyclopaedia or a similar book with chapters written by specialists on the individual religions would give much better coverage. And I wouldn't really recommend it as an introduction to the nature of religious belief either. It is beautifully illustrated, however, with many full page colour photographs, and would make a nice coffee-table book.