(a) a poorly patched extension used with Joomla 1.5. The Joom Credits demo on their site is still using or at least refers to Joomla 1.7;
(b) claims to be compatible with Joomla 2.5, but installing it through up multiple warnings for coding and SQL errors;
(c) has no user mannual or guide. There is no online documentation that I could find for this extension. You have to guess how to use it(assuming you can get it installed and working);
(d) support is poor. I sent an email setting the code errors and information about using the extension. Have yet to get a reply even acknowledging my email;
AND for those who may think my review is harsh, I went to the trouble of doing a completely fresh install of Joomla 2.5.9 and then trying Joom Credits again. Got the same (and other) errors.
A very poor rating is too generous (but there is not a "doesn't work at all and is complete crap" rating)
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).