As I mentioned a while back, I recently bought the Judges Guild Ready Reference Sheets. And I've spent the last couple week reading the sheets for the first time ever.
Some people have called the Ready Reference Sheets "the DMG for OD&D," and in one very literal sense, it is. Judges Guild had official permission to reprint some items from TSR books, so the sheets include combat and saving throw tables from Men & Magic, the weapon vs. armor class tables from Greyhawk, and pretty much every D&D monster to date, including some from the Strategic Review. The monster listings are in an unusually small font, which is one complaint I have about the product: they change the font size in several places solely for the purpose of fitting them on one page, without any regard for usability. I can't read the monster listings without a magnifying glass. And these aren't the worst: the treasure type table, which takes up a 6" x 9" page in Monsters & Treasure, takes up only 1" x 3" here.
The Ready Reference Sheets are also a "DMG" in the sense of being an eclectic mix of miscellaneous rules and tables for specific situations, some so specific that they may not arise very often -- but when they do, you're ready for them. A good portion of the rules are more fiddly than I personally prefer, and there's some bonus inflation creeping in: the magic item creation tables include +5 weapons and armor, for example. And the magic item tables also suffer in that they reduce the manufacture times dramatically: 4 weeks for a +1 sword seems far too short.
There are a couple abbreviations that are unclear. For example, the social level tables include two columns labeled "LVL GL" and "Carried Gold GL". The social rules also use this abbreviation, but it is never explained directly. In a completely different section of the rules, however, there is an indirect definition: the rules for limited wishes state that they cannot be applied to "starred GLs", and the accompanying table has starred items in a column labeled "General Guideline". So, "GL" probably means "guideline".
On the other hand, there are weird little items buried in a couple table entries that are intriguing. The tables of sharks, sea monsters, and fish are a little too detailed for what are, mechanically, mostly the same creature. But the kraken is listed as having six heads and 1d6 tentacles; the sunfish has "telepathic emotions" (does it project emotions, or does it sense emotions of others? Either way, that's an unusual fish.) There's a list unique trees in the tables for mapping wilderness areas, too.
I don't think I'd use much of the Ready Reference Sheets as-is, but there are many inspiring bits, and several sections I want to adapt into my own versions. For example, I may do something with social class, creating what I would consider to be a simpler, quicker approach. I've already done something similar with the posts on poison damage and poison effects. I may also investigate the random dungeon/cave generation tables to see how they compare to other random systems; The JG random dungeon table is the only one I'm aware of that distinguishes the physical construction of a level based on the creature that built it.
I'm sure I'll be returning to the topic soon.