I ran this scenario for a bunch of friends tonight with fantastic results. My group was sort of resistant to following the missing boy thread at first, but I managed to get them back on track. I think the focus drifted because I had given each of them one of the strange tidbits to chew over--like the mysterious bottle display, and a bit I created especially for one character who was an author where he walked into a bookstore and found copies of a book he hadn't written yet--and they were trying desperately to connect them. After a few reminders, though, Carcosa worked its maddening magic.
I think one of my favorite ways in which my runthrough varied from the one Ross ran followed immediately after they captured the kidnapper. My players wanted to interrogate him using the Good Cop, Bad Cop method, and didn't seem interested when McCready asked them to go up to the roof with him. When they told him they had to interrogate the prisoner, I had him ask them if he could sit in, and magic happened. Of course, he brought along a shot of sodium pentothal and a case full of torture instruments, and they bloody freaked out! They finally managed to make McCready stop doing terrible things to the kidnapper by convincing him that the usual methods weren't working, and he excused himself to go get a car battery and a sledgehammer.
All told, a great AP and a fantastic one-shot to run for your friends on a Saturday night.