I have had a Smart Case for roughly 9 months now and it has yet to get loose.
It can tear over time if you use the iPad a lot and are careless from time to time...but that is true with any case. Mine hasn't.
Contrary to popular belief, no color is dirt repellent so the quantity of the dirt will be the same. With that said, I find that it collects much less dirt than cloth cases because it just slides off.
NOTE: I assume you are talking about the Polyurethane Smart Case.
Overall, I have used better cases and I would not recommend this case to anyone that intends to stop any damage other than small rocks scratching stuff. It will not do much with a drop but it is nice that it covers the iPad corners.