So I know I'm late to the party here, and lots has already been said about this, but I just got my copy yesterday. I'm reading through it now and plan to post a review soon. Just wanted to give everyone a heads' up. I understand that Mr. Becker is sold out of these and is awaiting a second print run. If you can pick this up and are a fan of Moldvay/Cook or Labyrinth Lord, definitely pick it up.
So far, I have to say that it far outshines the version put out by Barrataria Games, which is too much of a move towards AD&D, includes a few too many d20-isms (such as adventuring kits for equipment), and is FAR too tied to Barrataria's campaign setting. That's not to denigrate their effort in any way; I own it and rather enjoy it;it's got some good stuff and would work well and serves a very worthwhile purpose as a Labyrinth Lord supplement. And while it is tied to Barrataria's campaign setting, these elements are easily divorced from the rules therein, so it's definitely and eminently usable in a generic sense. But it doesn't have that free and open feel that the original B/X rules had, and as a "true to the source" Companion effort, Mr. Becker's far outshines what is out there to date.
Detailed review forthcoming.