Even though it's a beta playtest version, it's very playable and presents a framework for numerous challenging adventures. The quality is that of a final product, pleasant to read, clear, structured, with just the right depth. The book is rich with gentle touches that you won't notice but will miss in many other small products.
I never played Original D&D and so can't compare the game to OD&D or verify their compatibility, but I see many attractive differences from other old-school games. One major benefit is selection of races and classes - not the vanilla human-dwarf-elf, but ten races from both sides of the usual moral scale; as for classes, there are also ten, including such picks as cultist and knight (inspired by Medieval more so than pop culture). Races and classes are written to really act their concept: for example, the knight lowers foe's morale rolls; the elf can't be surprised in forest setting; and the cultist perceives others' willpower, which is useful for interactions and combat. All the different things classes and races can do is a rich source for adventure ideas. When you have lizardmen who are great swimmers, you can have flooded dungeons. When you have knaves and robbers with black market connections, you can have black market.
The game gently stirs the game onto treasure-hunting, problem-solving, dungeon-raiding track. There's no focus on Good vs. Evil: "evil" races are not really different when your goal is surviving the dungeon. A wild random encounter with said races doesn't have to lead to bloodshed - with luck it can lead to a retainer contract. There is a solid framework for non-combat interactions and dealing with various hazards, and the combat itself is simple enough without exotic abstractions. The setting is technologically primitive, with no high-level NPCs and grand cities to take time from basic adventures. With a hint of forgotten apocalypse, it provides great potential for glory and treasure.
All and all, I think this game really deserves some glory among retro-bunch.
[5 of 5 Stars!]