Dark Dungeons is a pretty good retro clone of Rules Cyclopedia and BECMI combined. One of the things purists would have to watch out for is the way it doesn't mention that in the original rules, some classes had level caps. All classes go to level 36 and eventually get 2's in all Saving Throws in Dark Dungeons. Additionally, the Immortals rules are slightly altered and the paths aren't included, but the 36-level scale introduced in the Wrath of the Immortals boxed set is. But, for $25, it's a nice system for what it's worth. I'm working on doing my own retro clone of that system, but I'm hoping to throw in some extras, like a way to make new character classes, rules for monsters as playable classes, etc. Dark Dungeons included some systems external to the RC/BECMI books, but left a lot of other fun add-ons out.