This is a great little rule set, designed to be simple and portable. I love the idea that a core rulebook can fit in my back pocket. There are other simple rule sets out there, but the fact that Dungeon Delvers is designed for portability (it even provides a system for d6 rolls without die) means there's just no excuse to be without it.
I do wish the sources were included in the download; there are a few rules I'd like to clarify for myself, and I might even like to add a few attributes to character stats. It would be easier to extend if the source (maybe as a scribus document, or similar) were available.
Over-all, though, if you're looking for a quick and dirty ruleset that you can always have tucked in your wallet or backpack, this is it.