Edit: I should point out that the following is only my interpretation of things, not an "official" statement from Immersive Ink or anyone else involved with Delving Deeper. So if you don't like what I say, please complain to me and not to them, they already have enough work producing an excellent set of rules.
Over the past two weeks I've been "proof-reading" all three volumes of Immersive Ink's excellent Delving Deeper retroclone. It all started when I posted some feedback about the first volume over on the Original D&D Discussion boards. Simon Bull was nice enough to encourage me to finish my read-through although I had not initially planned to go beyond the first volume. But I have to say it was a fun, if at times tedious, experience. And I learned a lot about the rules in the process.
After the first volume I did change my attitude a little. Initially I read a bit in Delving Deeper, then looked for the corresponding info in the original game, then went back to Delving Deeper, etc. For the other two volumes I didn't really worry about OD&D very much anymore, I mostly focused on the consistency between the various rules and volumes of Delving Deeper itself. I should probably admit that I am rather OCD about stuff like that. I believe that a set of rules should be internally consistent no matter what kind of 40-year-old mess you're drawing your inspiration from.
But it's easy to do more than necessary, and Simon has an excellent point that weighs against my "grand ambitions" to fix everything: There are lots of things that are "exceedingly ambiguous" in OD&D that no matter what interpretation you pick (i.e. the one that makes the most sense to you), someone is going to be upset that you didn't pick theirs. So instead of a "true" retroclone that keeps the essential ambiguities, you'd end up with something closer to a "retroclone-ish" like Labyrinth Lord's Original Edition Characters or Swords & Wizardry's White Box rules.
Now of course Delving Deeper is not a perfect clone either. Thieves, for example, work quite differently from Greyhawk's OD&D thieves. Strangely enough, the variant that Delving Deeper presents actually feels more old-school than the original. Yes, in certain places Delving Deeper seems to "out-retro" even the original rules! Isn't that something?
I think my biggest pet-peeve about the rules is that a lot more things could be made more consistent without risking that "old school vibe" at all. So for example there are several monsters who "on 4 more than needed to hit" will cause some kind of special effect, swallowing a target for example. That's great, but those things are explained with every single monster and the explanations are not always totally consistent. For instance, some monsters get "on 4 more than needed (and on 20 of course)" instead. Which one is it? There are several examples like that and I hope that these things will be resolved with the next revision of Delving Deeper.
But aside from my pet peeve for consistency, there's nothing wrong with these rules. In fact, they are my favorite reinterpretation of the original game currently in existence, presumably exactly because they strive to stay close to the original and only go a different route when there's both a clear benefit and no chance of losing flavor.
If you haven't done so before, I highly recommend you take a look. They'll also be coming out with a sexy hardcover edition sometime this year! Right now you can grab the current version of it all for free right here. Maybe you'll even get hooked? Enjoy!