So... I bought this for a friend. As a gift. My friend trains Chinese boxing.
We filled it up with beans in the top, rice in the middle and sand i the bottom.
First of all, you'll find that you need a lot of sand, rice or beans to fill the bag. And you'll find that it's very hard to get it even in top and bottom.
My friend experimented with different fillings and eventually got something that's perfect for him (he mixed beans, sand and rice).
The quality of the bag is good and it seems like it's able to hold the sand in... There's zippers in the back and as long as you're a bit careful when pouring sand in it (so that the sand doesn't get in the zipper) it works very well.