Yesterday my Sunday group successfully completed Curse of the Crimson Throne after 34 sessions, levels 2-14, and almost 2 years of real time. The game blog is here - http://smonscurseofthecrimsonthrone.blogspot.co.uk/
Completing the AP feels rather like scaling Mount Everest - satisfying, but partly because of the sheer effort involved. Compared to running a 'normal' campaign, this felt vastly more arduous, reading all the books, trying to understand the intent of the series and the individual authors, working out what could be changed or avoided, what was the necessary backbone of each volume and of the AP as a whole. The backbone of a volume was sometimes hidden - eg in Book 5, the steps the PCs actually needed to take to complete the steps towards gaining the sword Serithtiel was buried deep in the text. In the case of Book 4 and its endless fetch-quests the whole thing proved unnecessary and I ended up not using most of it, making it a wasted purchase - but no way to know that in advance.
One conclusion I took away was that 3e/PF and its progression rate seems extremely unsuited to linear play with the scope of this sort of AP.
1. From the player end, PCs level up too fast and increase in power too fast, easily exceeding the credible scope - by the end of the campaign the godlike 14th level PCs felt like they should have been invading the Abyss, not merely saving a city. This would have been even more pronounced with the level 1-16 scope of the AP as originally written.
2. From the GM end, 3e/PF stat monster and NPC stat blocks go from reasonable at levels 1-5 to nightmarish levels 11+. Just reading stat blocks, looking up spells and powers, trying to grok authors' intent re battle tactics, took much more time than I normally need to prep a session, and this is just one part of running an AP.
The advantage of the AP was that it gave a satisfying, close-ended campaign experience, like completing a well made video game. I do plan to run APs again in the future, though next time I'll have a clearer idea what I'm getting into. I don't think I'll use 3e/Pathfinder again though. 5e D&D looks well suited to most Paizo APs, with 4e D&D also a possibility given plenty of editing to remove 'trash fights' - run as-is 4e would be much too slow. I'll need to be prepared for the extreme effort involved though; compared to running a more open or sandbox game, successful prepping and GMing of an AP takes far more work - which feels like the opposite of the common stereotype. Partly this is the time spent reading the text, partly it is the need to ensure it doesn't feel constricting and railroady.