Very, very page-turning. As a former (and recurring) D&D fanatic, I can say these books are straight up my alley. My familiarity with the fantasy setting of the Forgotten Realms, and the world of the underdark, are the foremost reasons I can't put these books down. But I'll try to look past that and assess its more accessible qualities.
Salvatore's characters are very real. Reading the adventures of Drizzt and his companions, and the exploits of his evil family and other denizens of the underdark, make me feel like such individuals actually exist somewhere, and the book is describing real, historically verifiable events. Of course this is quite a feat considering none of the species actual exist. Salvatore also gives the reader a glimpse of the (sometimes alien) mindset of these fantasy races - after a few chapters of the first book I began to think like a devious, power-hungry drow.
The author also describes the world in vivid detail, sometimes painting brilliant scenes. His descriptions of the characters' sight in the infrared spectrum make the underdark a colourful place to be - body heat radiating a bright red glow against the dark blues and blacks of cold background stone. He really squeezes a lot of life from a cold, dark, and empty wasteland, too, describing underground lakes of acid, glowing spore-spitting fungi, forests of mushrooms and moss, great cities carved from stalagmites. The monsters and other beings lurking in the tunnels are a treat, and Salvatore never goes too long without rewarding his readers with a chance encounter, a fierce battle.
Which brings me to the fight scenes. The more action going on, the more finely the author details, and combat is where he uncaps his thinnest sharpie. Every second of battle is painstakingly drawn to perfection, each enemy's dispatching painted starkly in the reader's mind. One gets the feeling of watching the fight play out in one's head, watching the whirling, thrusting blades and the flailing, soaring, crumpling bodies as they pummel and hack each other, gritty and raw, for survival.
And then the book has its tender parts too. I won't go into detail because I'm running out of steam, but parts made me cry, and more parts made me shout at my book, in joy, or hatred, or encouragement.
The book isn't perfect. The pace it sets is so universally action-packed, with fight scenes stretched out to entire chapters and (literal) years of uneventful wandering left to a few sentences, that the book seems unbalanced, disproportional, at times. It can be hard to conjure a mental picture of some of the locations and battle movements; there will always be some ambiguity when the writer doesn't care to bore his audience with an instruction booklet play-by-play. Readers without a fair knowledge of swordfighting technique will sometimes wonder what they just read; I occasionally find myself substituting his precisely choreographed combat with a pre-recorded generic fight scene in my mind's eye. Finally, the editing could use some work - run-on sentences and sentence fragments litter the books.
All that aside, a stunning and marvelous body of work from R.A. Salvatore. I look forward to reading the rest of his works set in the Forgotten Realms.