Every gamer who cut his or her role-playing teeth on the Moldvay Basic edition of D&D knows that the promised Companion Set detailing character levels 15-36 and all the epic adventuring rules to go with them never actually materialized. Instead, the game got a reboot with the Mentzer Basic and Expert sets, which were then followed by a Companion and a Master set. The Mentzer rules are mechanically very similar, though not quite identical, to Moldvay/Cook (or B/X), but differ considerably in the overall "feel" of the presentation. It wasn't perfect for fans of B/X, but for a long time it was the best we had.
Now we've got a couple more options, courtesy of the OSR, that purport to fill the niche of a Companion rules expansion. The Companion Expansion: Characters, Spells, Monsters, and Magic Items by Barrataria Games and The B/X Companion by Jonathan Becker, a.k.a. JB of the B/X Blackrazor blog. These two supplements, despite their similar names and some overlap, have significantly different emphasis. I realize I'm pretty late to the party here, but I thought I'd weigh in with my assessment of the pros and cons of each, as compared to the official Mentzer edition Companion.
The official Companion Rules for D&D: The Mentzer Companion set includes rules for advancement to level 25 for human characters (levels 26-36 were held back for the Master Rules set), several new sub-classes available to fighters and clerics at 9th level and higher, high level combat options for fighters, rules for high-experience dwarves, elves, and halflings to continue improving beyond their formal level limits, some new weapons and armor, new spells, new monsters, new magic items, a system for mass combat, a system for ruling a dominion, rules for unarmed combat, and a brief section on adventuring in other planes of existence. Most of this is simply extrapolating from and building on the preceding rule sets. The major exceptions to this are the War Machine mass combat rules, the unarmed combat rules, and the dominion rules, all of which (apologies to Frank Mentzer) feel wildly out of place in the classic D&D game. In particular, the War Machine and dominion rules manage to be vastly more complex than typical for the game, yet frustratingly vague in certain key areas. There's a tremendous amount of prep necessary to calculate the strength of forces in the War Machine, for instance, and a boatload of math to do on the fly during play, but how long is the battle represented by one roll in the War Machine rules? The book doesn't say. The rules for dominion accounting and confidence checks are similarly over-detailed yet nebulous in critical areas.
Companion Expansion: Characters, Spells, Monsters, and Magic Items: Pretty much what it says on the cover. Barrataria's Companion features tables taking the core human classes to level 36, several new classes, new spell lists, new monsters, and new magic items. New skills are provided for high-level thieves, since their basic abilities top out at level 14 in B/X and its clones. There are no options for demi-humans to grow beyond their formal level limits, although halflings in this supplement go to level 12 instead of the more traditional level 8. Additional demi-human classes include gnomes, wildwood elves, half-elves, half-orcs, and half-ogres. Added human classes include the illusionist, druid, and bard, fairly close analogs of the AD&D classes of the same names, and the scout, an approximation of AD&D's ranger.
The new spells section includes entirely new spell lists, one for illusionists, gnomes, and bards, and another for druids and wildwood elves. Most of the new spells are simply adaptations of spells from the AD&D game. It's still useful, in that it adds spells that actually fit the flavor of the classic (non-"Advanced") game, saving you the work of poring over spell lists and descriptions from various sources, digging out the good stuff, and compiling it in a single list.
The monsters section likewise draws heavily on AD&D, as well as creatures from classic D&D adventure modules (or perhaps the Creature Catalog.)
I'm not so knowledgeable about AD&D magic items, but I suspect that the magic items also draw on material from that game, plus a few I recognize from The Book of Marvelous Magic.
Despite the heavy reliance on AD&D as source material, the whole thing has a pretty strong Classic D&D feel to it, and I wouldn't hesitate to use most of the material in my own game. Since it's available as a free PDF, there's no reason not to at least check it out.
One of the appendices has an alternative combat table that slows advancement at higher levels by reducing the steps from two points at once. The upshot is that THAC0s for all classes bottom out at 5 points higher than they would otherwise. This is a nice addition for groups interested in high-level play but without quite so much power inflation.
B/X Companion: I could justifiably say here that I've saved the best for last. No new character classes are included (save the optional bard class presented as an example of how to add classes to the game), but the core human classes are extended to level 36, with multiple attack rules for fighters, advanced skills for thieves, and high level spells for clerics and magic users. Demi-humans get what amounts to a more streamlined version of Mentzer's system for continuing to improve beyond level limits. I've always found this to be kind of a silly workaround - it would make a lot more sense in my opinion just to raise or abolish level limits - but given that JB set out to bring the B/X game to its promised pinnacle rather than to create a new system or an updated clone, it makes sense that he takes pains to avoid contradicting the canon of those earlier rule sets.
The spells chapter includes some Mentzer-edition clones and a few welcome AD&D retreads, but also has several additions that to the best of my knowledge are original to the game, including a few that are at least hinted at in the official rule sets and thus long overdue.
Monsters are a melange of AD&D adaptations and new creatures, drawing from literature (balrogs, in the guise of the Bane Lord, black orcs, the terrible Jabberwocky and its fell compatriots from the famous poem), folklore (leprechaun, wendigo, banshee) and more modern inspirations (quicksilver golem and plague zombie.) There's also a new sub-category of monsters, the Greater Undead, which are immune to turning. Now you can safely use an undead creature as a Big Bad in your campaign without fear that it will be anticlimactically reduced in a single round to a smear of soot on the dungeon floor because a high level cleric waved a holy symbol in its face.
There's also a goodly selection of new magical items which nicely fit the B/X feel, and random tables that integrate them with the items from the previous rules.
Where the B/X Companion really shines, though, is in the new rules for mass combat and dominion rule so essential to really epic high level play. In sharp contrast to the baroque kludges of the official Companion Set, JB gives us a simple, abstract, and versatile mass combat resolution system that relies on existing statistics - Armor Class, Hit Dice, damage rolls, and Morale scores. No more futzing with Basic Force Ratings and Battle Ratings and how many weeks the leader of the force has spent training with the troops. The math is only slightly more difficult than that of normal D&D combat. Modifiers for tactics and special battle conditions can be applied as simply as you would a situational modifier in standard combat, as a +1 or -1 to the damage rolls used to resolve each battle. No more guessing about the time frame, either - mass combat is resolved in "Clashes" equal to 6 standard game turns.
On the dominion front, the rules for dominion population and income are extremely simple, and confidence is checked with a variation of the standard 2d6 reaction roll. One seriously wonders how such an obvious existing mechanic was passed over in favor of the weird agglomeration featured in the Mentzer Companion. There are even rules for how many peasants can be levied from the local population in times of need, an unfortunate omission from previous editions.
In the tradition of the previous sets, there's also a fair amount of miscellaneous advice on running the game, none particularly ground-breaking but all generally sound.
I found the $12.99 price tag for the PDF a bit steep, but having read it now, I don't regret buying it at all.
All three books have their strengths. For my own game, when and if it reaches those rarefied epic levels of play, I'll probably use the B/X Companion as the core, drawing on the others for additional content like monsters, spells, and additional classes.