Well, I haven't run DoMM but I've done a lot with Undermountain over the years and have read DoMM (and am working on incorporating it into my own Undermountain).
It's good and bad... There are a few things about it that I personally don't like.
- Halaster; I've always been annoyed by this personality, but the fact that he has been retconed alive after he has officially been dead annoys me to no end. I get that he could be used in a fun way, but in most cases I see this overarching all powerful evil but somehow benign or controlling NPC as such a crutch . Anyway, Don't like using him, so I leave him out of my version.
- Level gating; Some over powerful being that doesn't let the characters go to levels that are too dangerous for them. Why? Why would such a creature care? And, to me, levels are a metagame concept that beings in the fantasy world really don't have any concept of. But, this part can easily be ignored.
- The maps/layout; to me the Undermountain is huge. DoMM is not. rather than 300-400 locations per level, DoMM has 2 dozen? That's not the UM to me.
- Along with the level gating and the portals, to me DoMM is much too linear. Even though it is not linear, to me their should be more ways in and around and about. Of course, with 300 locations per level, you need more.
Now, on the good side, it does capture many of the aspects of the classic Undermountain. Such as weird traps, magical fantasy places, and danger. High risk, high reward.
So, my biggest issue is that it's not big enough, but if you want to put the time into it, you can expand upon it.