It's DocMan on steroids, has great features and the Documents Seller team is helpful as (special thanks to Tuan from Allen).
If you have used DocMan then much of its look and feel will be familiar. If not, then there is a learning curve, but worth the effort. The only negative I could see was a lack of documented instructions, but this is being addressed.
In short, if your serious about selling online then this is an extension you should take a good look at and consider the purchase (even if, like me, you don't generally buy extensions).