I received H.P. Lovecraft: The Complete Fiction as a gift from my son; it is truly one of those gifts that keeps on giving.
The brief introduction at the beginning of each piece by editor S.T. Joshi was a nice addition. Generally a short paragraph giving a bit of background on the piece, it helps place the work in the continuum of Lovecraft's writing career. I did not feel that they contained any major 'spoilers', but rather discussions of stylistic forms, date of first publication, and notes of interest such as rejection by a certain publisher. As a novice Lovecraft reader I found them helpful.
The book takes you more or less chronologically through Lovecraft's work, rather than grouping the stories by style or subject. I enjoyed being able to see how the author evolved over the course of his apprenticeship into the craftsman he was to become. You can see how other authors influenced him, and how he truly learned by doing, integrating ideas from his previous works into what became the Cthulu mythos.
This is a heavy book, in subject as well as mass, so it's not one to read nonstop in the course of a few days. This has held the place of honor on my nightstand for months, to be tasted in discreet and enjoyable morsels, mulled over and digested, then tasted again. It has certainly brought some interesting color to my dreams.