I loved the 8 hours it took for me to beat the game. Good length for a $20 game. But I too felt the ending was weak. In Penumbra Overture, it ended with the Philip burning Red alive then getting knocked out. In Penumbra Black Plague, it ended with an encounter with the Tuurngait creature and Philip refusing to follow it's wishes. Both had clever, satisfying endings. But Amnesia was extremely anti-climatic at the end. All the build up, then I walk into a room and touch a few things and it ends. No clever twist, no emotional connection. It's like the game just stopped.
edit: on second thought, maybe I just didn't want the game to end