The Merck Index is probably the only reference work of its kind, that is, a one volume description of almost every single chemical that would be encountered by all but the most esoteric of professional researchers. As such, it is a good book to have for anyone in a scientific profession, including medical professionals.
It it also a very unattractive and unwieldy book, and is the type of science book that scares many newcomers away from chemistry. Part of this is unavoidable: there is no way to describe so many chemicals and have a book that runs shorter than about 2,000 pages, and there is also no way to describe chemistry without much arcane terminology. And in a (unsuccessful) attempt to make it shorter, most of this terminology is in abbreviations. Also, the amount of information that is given for entries varies quite a bit, with some having detailed information and structural formulas, while others have a much simpler empirical formula. Some entries describe what a substance is soluble in, and some give exact ratios. There is also usually a long string of numbers describing the patents that apply to the substance. Simply paging through it can make one's eyes swim.
So the Merck Index, while it is quite useful to have, also has a problem that it admits right up front: it is an index, a thick reference work that serves up lots and lots of facts, and the context for these facts is something the researcher has to work out on their own.