ACG is a moderate-sized piece, well-tested, with a wide variety of responses to unusual conditions. It's unabashedly a puzzle game -- the premise is a bit thin and the story is minimal -- but what it does, it does very well. The puzzles are generally fair, and many are quite ingenious: the objects you've been given at the beginning of the game can be used in a satisfying variety of ways. Very few of the puzzles felt at all shopworn or perfunctory.
The attention to detail is also excellent. There are a number of easter eggs and special endings -- while there's only one way to really win, the alternate semi-loss conclusions are great fun to read.
One thing that many players are likely to find surprising is the absence of response to EXAMINE: Thorsby eschews object descriptions entirely. Everything you need to know about a thing will be evident from its room description and inventory listing. (On the other hand, this makes for some very long inventory lists...)