I really enjoyed it quite a bit when I played it a while back. I especially liked Gorrister and Nimdook's chapters, while Ellen's felt the weakest, but was still interesting.
I remember the game suffering from adventure game logic with one puzzle, but a bit of trial and error solved that easily enough, and I remember most of the other puzzles being reasonably intuitive. The twistedness of AM and the different environments makes it pretty entertaining from start to finish, as well; each character also has their own issue, shortcoming or phobia that AM tortures the victim with, so that psychological aspect is also interesting to see incorporated into each character's sub-world.
While the game directly quotes the original short story at certain points, they share quite a few differences. Both the game and the story are well worth checking out, I'd say. I also find it pretty neat that the writer himself, Harlan Ellison was the voice of AM.
Also, here's the game's intro for anyone who wants to see a little of what the game's like:http://www.youtube.com/watch?v=iw-88h-LcTk