Let's start with the business you build up from failure to success, sending the players every signal that they should have their PCs put down roots. They have awesome support for that business: floor plan, descriptions, secrets, and easy rules to improve it. Wonderful city filled with hooks too. Then you leave it in the second module and never return.
Then there's the idiocy at the end of the third module, where I had a long argument on the boards. (
paizo.com - Paizo / Messageboards / Paizo Publishing / Pathfinder® / Pathfinder Adventure Paths / Second Darkness Adventure Path / Winning the Battle for Celwynvian (spoilers)) Basically, I don't understand why any tactics-minded player group would send his PCs alone into a different dimension to face an entire army to save a jerk and a scout they just met.... after facing a drow squad, an aboleth and a barbarian back to back to back. It looks exactly like a suicide mission, which it should be if the cunning drow weren't so stupid as to leave not one single person on the other side of the portal. Worse, it's a suicide mission when there's clearly a much better option available: sit tight with a superior forces, reinforcements, and supplies.
Oh, and did I mention that they might not have Plane Shift when making that decision?
Then the fourth adventure came out. Basically, the party goes undercover in a drow city, literally black face. While that could work out Jar-Jar-Binks-does S&M-bad at a table, if it goes okay... the DM gets the work of designing an entire drow city to explore, albeit with SOME help from the module. But, basically, it's the most labor-intensive of all the modules, for kind of minimal gain. I forget. I honestly checked out on that path by the end of that module.
As always, there's good parts to pick in the series... but I can't say that I recommend it.