It really, really depend on what it is, for me.
If it's a "setting book", so when compared to D&D, includes PH, DMG and some monsters (no setting book I've ever seen has as many monsters as a MM), I don't like it, really. Either you have only one book for the group, which makes it a pain to make characters, and to run the game if you need info from the book (for monster stats, for example) at the same time as one (or more!) player needs info on a spell. Or you have your players buy a couple (or one each), which means they spend money for information they don't need, and that, frankly, I'd prefer they don't -have-.
If it's a "source book", then it really depends on what it's a source -for-. Only if it's for something that will see -lots- of use in the campaign will it be a good investment. In comparaison, a smaller (and cheaper) book on a same subject would probably be a good investment even if the subject only see moderate use.
If it's an "adventure/module", IE WLD, then it's good. I bought the WLD, and I'm really pleased with my investment (granted, I got it for 40$ at Amazon. Prolly wouldn't have bought it at the list price, since I couldn't be -sure- it'd turn out as great for my own need as it has). To me, it's better than getting a bunch of separate modules, since those, even if part of a "serie", will leave me too much work to do to "link" them, for my own tastes. After all, I bought the stuff 'cause I -hate- prep work (not the reading part, but the writing up stuff part), and have lil time for it.
So, really, it depends.