Good points are the variety of environments they travel through - lava tunnels, caverns, mine train, lake, swamp, island, bunker, ruins, with the promise of more to come. The variety helped maintain interest; being realistic about being lost in the dark added a great touch of realism and made me realise that there was no guarentee that the heroes would survive unscathed; characters are kept few and interesting with a few surprises.
Bad points include the length of the book - at nearly 700 pages it seems indulgent for the author and takes a good few sessions to wade through; the ecology, which seems very random and extreme at times - most areas are desolate and rocky, whilst others have a variety of life, but then some teem with giant animals that seem to have no food source (the cave cows) and no predators or competitors. It just felt too artificial; plants with no energy source supporting them; unstoppable and innumerable foes - endless ranks of limiters and styx, robotic and unbeatable.
All in all, a good adventure with plenty of variety and story twists with intrigue and danger in abundance. I'll certainly pick up the next one, though I'll enjoy a few shorter books first.