Asolutely love this. Whilst being a tome that ties in nicely with the Old School Essentials set, this is usable with most B/X (or even later Mentzer/Cyclopedia) versions of Basic D&D. It introduces race & class as distinct whilst keeping the flavour of osr gaming (you have the option to keep race-as-class, and it includes many new additions for this style if you prefer it).
I particularly liked the feel of old-style classes (Acrobat, Assassin, Ranger etc.) in the context of the Basic set-up. Also includes 'advanced rules' (two-weapon fighting, parrying, multi-classing, skills, weapon proficiency), yet somehow manages to 'feel' like its own thing, i.e not just a compromise with AD&D rules tacked-on.
I find the rules herein far more intuitive than the weapon rules offered in the Cyclopedia version of basic D&D (which add more complexity than AD&D in many ways, albeit adding great depth).
OSE keeps to the design principle of 'rules-light' and intuitive, whilst serving up something very evocative of what I had been looking for whenever I went back to try the 'basic' version of the 'Original Game' and found it missing something. This volume is an incredibly good answer to that feeling of what was missing. Great work, all!