I cannot speak highly enough about this series. I own all three volumes. This was the series I was working my way through in high school - and I never realized how much I had really learned from them until years later.
Bad art and bad jokes are the credo of an foreign language text worth its salt, and this is no exception. However, the exercises are clever and engaging. The grammar is simply and plainly laid out. I love the way that each chapter explores a particular topic of grammar very deliberately and slowly - for example, an entire chapter devoted to the concept of the Preterit.
This is a book I find myself returning to as an adult learner returning to Spanish, when my adult-level text gets a little to difficult or confusing for me.
This book is everything I would need to teach an entry-level Spanish class on my own.