I just picked up a fre for my 5S last Thursday.
So far it's been in my pants and hoodie pockets and it hasn't let any dust or debris inside. Seems completely dust proof. I even opened it and took my 5S out last night to check, and not a single spec of dirt or dust.
This is mostly why I bought the fre. It's expensive, but I hate lint and dirt getting jammed into the speaker holes or earpiece grill from pockets. This case 100% prevents that. I'm gonna keep my eye out, because the nuud is coming soon. Lifeproof is hands down my favorite iPhone case. Adds relatively no bulk and protects your phone from the elements. I love being able to sit my 5S on a wet counter, or throw it into my pocket with keys or change. For someone who is OCD about my phone, it really gives me peace of mind.