Dark Dungeons is only kind of playable, but that "kind of" is miles ahead of many other games.
I say "kind of" because Dark Dungeons suffers from really bad layout that blends sections and subsections together to form one giant wall of three-columned text. This is almost a fatal flaw in the book. I don't blame the author of the text, since he's imitating the original Rules Cyclopedia, but I do wish it were broken up a little better. The book really, really needs about five to ten pages worth of white space inserted into it, as well as a different set of fonts to mark out subdivisions of the text more clearly. I also think a two-column organisation would help the readability.
Other than that, Dark Dungeons is the Rules Cyclopedia with a few tweaks and updates, almost all of which I like. The Rules Cyclopedia was the only single volume edition of Dungeons and Dragons ever published, and it was the version I started playing D&D with. While I think it's a little more complex than my current players would prefer, I think there are many spot rules and tables that I would gladly pull out for use in my Swords & Wizardry Complete game. The gun rules, the rules for traveling between celestial spheres, for building strongholds, assembling armies and leading them against opponents, I love all of these things.
I'm not sure if I can put my finger on exactly why, but Dark Dungeons seems to evoke a much more vivid feeling of potential adventure than many other retroclones by veering away from the medieval milieu into the sheer fantastic. Unlike many other retroclones and versions of D&D, where the end-game is very vaguely defined, Dark Dungeons gives me lots of ideas for how to have the PCs become nobles and eventually, even gods.
The game is best served by having a referee with strong organisational skills and a familiarity with the book who could guide players through the process of creation. There are a lot of tables to be read off of, but only a few to be referred to. Post-It notes are a must for quick reference, especially for attack actions, calculating initiative and damage, and a couple of other fiddly details. I initially thought that the game's use of ascending base attack bonuses and descending AC did as well, but it turns out that one simply adds the two together to form a single bonus to the d20 attack roll, and if the total is over 20, one hits.
I'm thinking of creating a setting specifically to use for a Dark Dungeons campaign, probably oriented around traveling from planet to planet. More on that when I've thought it through.