Before They were Giants is an anthology without a unifying theme, a collection of the "first published" works of a wide range of living SF writers. There are 15 short stories in about 200 pages, which includes a brief questionnaire with answers by each author (itself generally two or three pages long). As a result, there is a real grab-bag of things here, with "sci-fi" being pretty widely interpreted and the authors involved ranging from Piers Anthony to China Mieville.
As a result, by operation of the simple law of averages, there are bound to be a few things here you like. The bad news is, its probable there will be quite a bit more you either dislike or are indifferent to. Its also billed as a primer for aspiring writers - which it may very be of some use as, I have no idea - but as a result it also straddles into teachiness in places at the expense of fun. Add in the fact that this is, by definition, the rawest works of the authors involved, and the book becomes of more use as a curiosity striving for "importance" than a book trying to be "fun".