I mentioned a nice compilation of the OD&D rules here a few days ago (see The Gray Book: OD&D Compiled). I printed out a copy and the players at my Sunday Microlite74 game looked it over while we played.
The general consensus is that The Gray Book is the best "revised" version of OD&D and supplements yet. It is well-organized. While it incorporates a lot of text taken straight from the various OD&D booklets, it clearly written and easy to understand. Part of that clarity comes from having everything related to one subject in one place instead of scattered across many booklets, but the editor added additional text here and there which simply explains things better.
There is one major flaw with the system, IMHO, it uses the Dexterity-based initiative system from the Holmes Basic Set. I've never liked this as it requires the GM to roll (and track) each monster's dexterity. As far as I'm concerned, that's far too much work for too little gain. However, it is easy to substitute a different initiative system into the game.
The Monk character class from Blackmoor and psionics and artifacts(from Eldritch Wizardry) are missing from the rules, as are some optional rules that few people used such as weapons versus armor class and hit locations (from the Blackmoor supplement). Illusionists and Rangers have been added from OD&D supplements, and articles in the The Strategic Review and The Dragon. The Ranger also has a lower-powered version of spells for Rangers from AD&D. A few other minor things have probably been taken from AD&D as well, but for the most part The Gray Book seems to have drawn on OD&D. It take little effort on a DM's part to add material written for either OD&D, B/X, or AD&D 1e. This makes most of the material from early issues of The Dragon and White Dwarf or from modules by TSR or Judges Guild available.
This will probably be my "go-to" version of OD&D for the future. It will be easy for players to print out if they want a copy. It uses OD&D hit tables and saving throws. It includes the classes and other material from the OD&D supplements -- or at least most of the material in common use. It is well-organized and easy to understand. Unlike Swords & Wizardry, all the advice on creating dungeons and wilderness adventures from the third OD&D booklet is in The Gray Book.
While the original OD&D booklets cannot really be replaced, The Gray Book comes closest to being a usable replacement, at least in my opinion. Unfortunately, it's definitely a "gray market" item. It's free but it is a definite copyright violation as, unlike the retroclones, it is not covered by the OGL. However, as I said in an April post, since WotC pulled the legal PDFs of older versions of D&D from the market, I no longer feel the need to play unpaid copyright police for them at my game table.