I finished the first couple of novels in the series, and got about halfway through the third, though I managed to collect every novel in the series. (I'm talking about the original editions, not the recent reprints. BTW: Do these reprints have numerous "explorer's journal" illustrations, as the original editions did?)
The overall premise of the "Dungeon" series is fantastic, and it would certainly make a good roleplaying campaign. However, I found myself losing interest in the novels after the second one. (That was, oh, 10 years ago, and I might not have been in the right mindset at the time, so maybe I should give them another go?)
The overall mood and setting of the novels remind me more of Edgar Rice Burroughs and H.G. Wells than they do what many today consider to be "heroic fantasy". The emphasis in these novels is more on mystery and mysticism than it is on action and adventure. There's also a good deal of exploration, though exploration more of alien worlds and exotic landscapes than of D&D-like "dungeons". And there is a liberal sprinkling of "steamworks fantasy" and "Victorian horror/romance" in these novels.
Comparable RPG moods/settings would be EverWay, Castle Faulkenstein, and the "Witchfire" trilogy of adventure modules by Privateer Press.