I read all the other orchestration books first, mostly because they were available to me in libraries for free. However, I wish I had been able to start with this one -- I would have learned much more quickly.
I dithered for about eight weeks before deciding to spend the $71 on this book and the $100 on the CD set (which must be purchased separately) -- and the two together have been worth it in every way. It's a wonderful text. You can't learn everything from one book, but this is one hell of a good place to start. The ability to hear various reorchestrations of the different pieces is absolutely invaluable, and no other book can provide you with this kind of instruction.
Initially I was scared off by some of the errors (of fact and of judgment) listed in the reviews below, but I was relieved to see that most of them had been either fixed or altered in the third edition. For examples, the word "blaring" in the Hindemith discussion (Ch.11) and the problematic trombone glissando citation are totally gone in the third edition.
The section on guitar, banjo, and mandolin is indeed inadequate as a discussion of those instruments, and I wouldn't use that part of the book if I wanted to write for those instruments. Actually, I can't think of one orchestration book, except maybe Blatter's, that does/did have an adequate discussion of these instruments, because they're simply not often used in the orchestra; if you need a thorough discussion of the guitar, banjo, mandolin, and all the rest of the guitar family, you'd probably be better off getting a book *devoted* to those instruments rather than an orchestration book. The guitar family is very complicated and has traditions behind it which are far different from those behind the orchestral instruments.
Incidentally, the range given for the classical guitar is not exactly *wrong*, though it is low (open 6th string to the 12th fret on the 1st string) -- but it is the range commonly given in orchestration books. Go figure.
I've read the book by Kent Kennan just about twice, and I recommend it for further reading before delving into the older books by Piston, Rimsky-Korsakov, and Berlioz-Strauss. The Kennan book would also make a fine supplement to Adler, because Kennan has a way of pointing out certain facts that give you a "key" to very complex situations; for instance, he points out that the most successful multiple stops on the violin are made of sixths, fifths, and open strings -- a fact which can free the student to write multiple stops with confidence (so long as s/he checks them out later on a fingerboard chart), and his discussion of the trombone glissando, at least, is better than the one found in Adler.
All in all, if you've got $80 to blow on an orchestration book (or even better, $170 to blow on book plus CD package), this one will be worth the money.