In my latest B/X game, I looked through some maps from Dyson Logos, picked one (I'll share which map once the PCs have finished exploring) and populated it with the Moldvay rules. I used the random encounter list for monster picks, but "improved" on the results with weird and different creatures. I riffed on the dungeon's contents to come up with a backstory for it involving elves and a crown, and off we went.
Once again I used Geoffrey McKinney's Dungeon of the Unknown for some of the oddities, including a glop and one of the chimerical beasts. (C15, a smilodon/scorpion creature) that was met with some ferocity from the PCs. They used fire to slay the glop, and chokepoints to fend off my own Devil Monkeys. The leaping capability of the Devil Monkeys made the initial encounter really dramatic, as they lept clear 15 feet over an underground pool to attack the elf who thought he was safe taking pot shots.
Moldvay's dungeon generation gives a good balance of rooms. I supplemented it with the AD&D DMG, the Dungeon Alphabet, Ready Ref Sheets, and Dungeon of the Unknown for inspiration. A mix of traditional monsters and new / weird ones hits my sweet spot for D&D play. There's new and challenging encounters, and ones where player familiarity kicks in, like the troglodytes the players have not yet taken care of.
The most striking thing about last night's game, though, was how my B/X campaign has worked out fairly well in terms of having a large player pool. In the previous session, several PCs moved on from the Keep on the Borderlands to a town upriver. But none of those PCs made it to last night's session; only the player whose PC had died in the Tower of the Stargazer was in common between the two, so the PCs actually were at the Keep at the start of the game. It's an interesting shift in dynamics. There are a total of 10 living PCs with experience in the roster. Several new 1st level PCs went along with a 2nd level character (dating back to the Stonehell game I was running last year) who made 3rd on this adventure.
One thing I've noticed out of this is that the PC group's capability is really driven by the level of the highest level fighters. Since they can take the front lines during a fight, fighters over level 1 really take first level encounters out handily. And of course, numbers still reign supreme; higher level encounters with one monster don't make as much difference as more lower level opponents.
B/X D&D is holding up pretty well. I've noticed it has just enough guidelines in the rules to be firm on certain things, but not overwhelming with detail like AD&D 1e can get. A brief discussion of river transport clarified that for me: it has what I want as a referee but no more than I need. It's really a good, straightforward implementation of OD&D in the ways that matter. In practice I am finding it adapts to what I want a bit more smoothly than either Holmes or Swords & Wizardry did. OD&D of course retains a special spot in my heart, but I have been happy with B/X on the table.