Parts of SoC were the best moments I've experienced in gaming. Delving into sealed-off bunkers or suddenly encountering snorks in the middle of the night with only a few bullets left was... An experience.
At the same time I feel that the series could have been so much more after two sequels. In CoP it felt a bit like they were going on rails, and the voice acting was getting on my nerves. "Spill the beans", anyone? It was good, but not great, and the game engine peculiarities started to seep through the cracks and kill the atmosphere somewhat.
I sure hope someone carries on this tradition of intelligent fps gaming, which is a rarity.