I ran the second and third books of this adventure converted to v3.5 (I modded the first so heavily that I don't think I can accurately review it).

Book II was excellent for the most part, really pulling through the underdark feel while having plenty of interesting scenarios. My gnome and elf players absolutely loved the Svirfneblin and Rockseers subplots respectively, and the different encounter areas along the path were interesting challenges. The City of the Glass Pool section was quite difficult, forcing stealth and hit-and-run strikes, but the party said they didn't really have a great idea of their objectives. This may have been my fault to some extent but short of just handing them the SCP scorecard, which feels blunt and not immersive to me, I'm not sure there was a great way to communicate that.

Book III was also very good, but felt a bit less directed. The party knew that they had to eventually attack Shaboath, but (wisely) spent a lot of time exploring the cavern looking for allies and resourced. While individually places like the Isle of Shadows were quite interesting challenges, some of them felt a bit disconnected from the aboleth plot. Still, they were great seeds for the campaign world in general. The question of whether or not to ally with the renegade illithids or tanar'ri was very interesting and led to significant debate within the party. The Great Towers were a very cool concept, but I thought the simple "burn X spell levels of the opposing school" solution was a bit boring (and also 3.x removed the 'fixed' opposition of schools) so I changed the requirements to shutdown each tower to be a bit more unique and that went over very well with my group.

I converted this to 5th edition which was rough but isn't too difficult with Kobold Fight Club to help. The story was well written and interesting. I can only speak on the adventure up until the beginning of the Garlstone Mine. The town is intentionally sparse in resources which makes it easier for the DM but also easier for the players to stay on track. The 3 different parts of this module are easily dropped into any campaign just with some name changes and encounter conversion.