Dark Dungeons is a great retro-clone. I like it. That said, I'd probably never run with it, as I still have my original Rules Cyclopedia and you can buy the PDF of it cheaply at DriveThru (like... $10 or something).
The Rules Cyclopedia was awesome. If you like B/X or BECMI D&D, it's the holy grail of D&D books. And Dark Dungeons was a great replacement when the only RCs available were going for $75 (at least) on eBay. But now that the RC is back in play it seems a bit redundant.
Oooh! Just looked at the DriveThru page for the RC, they're offering hardcover color books for only $30! I might have to get one.