I played both Amnesia and Penumbra (complete trilogy). They are very intense and horrifying games. In my opinion Penumbra is a bit creepier, but Amnesia has the better story and atmosphere.
I wouldn't say this games 'ruined' the horror genre for me. They had it's scary moments, but in other games like Silent Hill there were parts which scared the hell out of me, where the games from Frictional just scared me sometimes, and much less. Just a few moments when enemies suddenly rushed out of a corner really shocked me, but most of the time it was just running through dark areas and solving some puzzles of various difficulties.
But great games anyway, really worth buying!