If you're a fan of horror games yet you've somehow managed to avoid playing Amnesia: The Dark Descent, consider this your lucky day! Simply head on over to Steam and you'll be able to grab a free copy of both the original Amnesia and its sequel A Machine for Pigs. This offer will only last for a "limited time only", so I would recommend you seize it as quickly as possible lest you lose your chance.
As for whether Amnesia is worth your time, I would say without even a single doubt that the answer is yes! Unlike the vast majority of horror games it doesn't rely on jump-scares in order to make your pants brown, but rather on a combination of immersive atmosphere, truly disturbing visuals, and excellent sound design. So turn off the lights, put on some headphones, and just dive in head first - it is well worth it!
While I consider the original Amnesia to be one of the best horror games ever made, I can't exactly say the same about the sequel. It was made by entirely different developers, and even though it isn't as bad as people say, I didn't find myself enjoying it as much as I did the original. It's still worth a playthrough, especially given the extremely fair price of "free", but just don't expect it to live up to Amnesia's legacy.
If you need a little bit more convincing before you take the plunge, I'll leave you with Amnesia's original trailer. It's a little bit outdated by this point, but it should get the point across pretty darn well regardless. Enjoy!