Just finished Penumbra series and...

[Monkeyballs25]

I actually found the Amnesia monsters scarier because they didn't hang around for too long, but were much deadlier when they are present.

Spoiler below!

Once I figured out that you could outrun the Penumbra zombies indefinitely, they became a bit of a joke really. Amnesia's are so much worse because once they spot you, you're normally dead unless you're very close to a level exit door.