This could have been really great, the idea is both sound and really useful to both players and DMs, a volume compiling and organizing new play variants, classes, races and so forth into a playtested companion to the core AD&D rulebooks. Unfortunately the execution of this good idea left much to be desired. 

Gygax, with the help of Jeff Grubb and Dragon editor Kim Mohan compiled articles from the magazine and other sources as well as new stuff, playtested them, and put them in the book. All great up until now, however the book ended up being infamous because of the amount of mistakes which made their way into the final volume. Corrections and erratas would be upcoming, but all too late to save the reputation of this volume, which was meant to kick off the process for a second edition of AD&D, but ended up not being used in the next edition of the game at all.

As I say, this is a pity, there’s some really good stuff here, and if you use the later corrections and are wary of mistakes, there is plenty of useful material which can still be salvaged from the volume. If nothing else it brought us another iconic Jeff Easley cover and a title which would be synonymous with experimental D&D content up until our days!