I've been seeing a lot of mentions of, and material for, the Black Hack lately, so I thought I'd pick it up and take look for myself. In broad strokes, it's basically an adaptation of elements of White Box Swords and Wizardry, Dungeon World, Castles and Crusades, and D&D 5e with some house rules thrown in at critical parts. Surprisingly, it looks like this mix works reasonably well. This will be a review of the game with the Additional Things supplementary PDF available for free download.
Overall, I think the Black Hack is good, but it's good because it's a mix of some great and inspired ideas with some very badly done ones, rather than because it's modestly well-done all the way through. The basic mechanic is d20-roll-under-stat to accomplish anything, with PCs making most rolls. i.e. they roll to avoid monster attacks instead of monster rolling to hit them.
The great and inspired ideas tend to be little ones that aid playing the game. I particularly like the new time system (moments and minutes), the usage die for consumable items, the resting mechanics (an OSR adaptation of the D&D 5e rest mechanics), and the use of opponents' HD as the main stat you have to track. From Additional Things, the tags for gear, the rules for panicking if one runs out of light, and the coin-dice rules are all very clever. These are all solid ideas, most of which could be integrated into any retroclone you're playing, and would add a great deal if they were.
The bad parts tend to be the more slapdash elements of the book. Firstly, the gear list is incomplete, lacking 1H weapons or bows in the gear list. Similarly, it has different weapon proficiencies by class, but these are just random lists of weapons, none of which are mechanically different from one another. The morale rules and the reaction rules should be simplified into the same set of rules, and the old 2d6 system was superior to the one in the Black Hack. The fighter class needs its ability reworded (Additional Things proposes one fix, and I see other versions repeatedly proposed or discussed in the community on G+). The gear list is the same slight alterations on the basic D&D 3.5 gear list that everyone's been using for a decade and a half now. Probably the worst of these various rules is the one for random encounters, which assumes that you're rolling every 15 minutes of real-world time, which I think is just unworkable in most situations.
After reading it, my general inclination is that I like it and want to play it, albeit with some heavy house-ruling. The chassis underlying that house ruling though, is generally pretty solid. If you like universal mechanics, rules-light systems, and / or systems where PCs do most of the rolling, I think you'd like it as well. Even if you don't, I suggest adapting the usage dice mechanics, the coin dice mechanics, the moments and minutes time tracking, and the rules for panicking in the dark for use in your own retroclone games.