Thoughts, comments, reviews, suggestions, observations, rumination, primal screams?
It has almost become the gold standard of the OSR that has made a lot of other B/X retroclones obsolete.
What Aldarc said. B/X took over from AD&D as the most popular TSR version in the OSR some years ago, and OSE (originally named B/X Essentials) has become the most popular form of it because of its super clean and user-friendly layout and organization (as well as its ongoing series of high-quality modules). Between that and the sturdy and attractive physical books, it's super convenient and usable as a reference.
Unless you've already got a lot of experience with the original B/X books and know where everything is in them, OSE is the most usable and accessible way to play it.
And the Old School Advanced expansion does a wonderful job of back-porting much of AD&D's expanded content (classes, races, splitting class from race if you want, spells, magic items, monsters) into the simpler, cleaner B/X rules framework. So you get the best of both worlds.
I agree that buying just the three volumes, the Classic Fantasy Rules Tome (all the core rules from B/X) and the Advanced Fantasy Player' Tome and Advanced Fantasy Referee's Tome (all the expanded AD&D-style stuff) is the best way to buy it. You don't even need the Classic tome if you just want to use the Advanced stuff; the Advanced books include all the core rules content from Classic. But I like the option to just use the simpler rules all in one book.
The rules themselves are B/X without any substantive changes (just occasional editorial decisions where something was ambiguous), though, so they retain that edition's issues. Thieves basically suck, unless you house-rule them. HP are low and 0HP = immediate death, so that calls for a house rule at most tables.
My current favored house/table rules are
here.
