First off, this is the Version 1 WE STANAG magazine for their open chamber GBBR's, not the newer Version 2. That means you get all the design flaws of this magazine.
Out of the box, the two I ordered didn't leak. However, considering that the three I already have all had issues before I fixed them, we can assume that these will develop similar issues.
The biggest issue is the seal between the reservoir and the back plate, over time the plate can warp slightly and start to leak. This is a relatively easy fix using blue gasket maker, which I did before I even fired the gun with the new magazines. A guide to do this fix can be found using Google.
The second issue is a small metal plate underneath the feed lip, over time this will start to leak. It can be fixed using superglue or JB Weld around the edge of the plate, I would recommend the latter.
Also, you need to be careful if you ever decide to remove the fill valve, the stock ones are very weak and will snap if overtightened even a fraction of a turn. If this happens new valves are pretty much your only option, and the Madbull ones are much sturdier than the stock ones.
It doesn't leak out of the box.
It only cost $32.97 when I ordered it.
Can be changed to hold 35 rounds relatively easily.
Holds enough gas for over 70 shots on semi auto.
It will most likely start to leak shortly after you get it.
Fill valve is weak and will snap with barely any torque.
Internal components are pot metal.
Extremely poorly designed.
Rating: [2 of 5 Stars!]