It only slows you down a tiny bit each tier, until Level 20, when it's quite Feather Fall like. What it really basically does is cap the amount of fall damage you can take. My current build has stopped Monk levels at 12, and he takes -9 damage from any fall--well, or less if it's a short fall. Also, you can't toggle it off mid-fall (well, you can, but it won't change your rate of decent).
EDIT: Errm...Impaqt posted first...I swear...