This si a specialized piece of equipment at a premium price. it's not for anybody or everybody in general. It's a combination unit that needs to do all it's jobs and do them right to work for you. It DID work for me.
I got this for a custom M4 that would not take a Ready Mag on the JP reveiver. This ended up covering my entire top rail which did several things.
It raised my rail platform by about a half inch, stopped the RIS from wobbling and added a ready mag.
My unit however did not function as a propper ready mag out of the box. It grabbed my mags and would not let go, even destroyed one mag in the process. I had to disassemble it (painless, just grab a punch or awl for the release arm pin) then snip down the spring for the mag catch arm until the arm had enough room to move out and let my mags come away freely as normal. (this was a better solution than grinding down the metal corner of the mag catch)
The mag well is still designed with the AEG version in mind. They give you a couple little squares of foam to add some cushion to the thing, but these are more for not smashing your feed tube when you slam a mag up into the well. You can push mags all the way to the top very easily but they won't hold tight for long. The mag catch holds them in the well but not securely, they wobble terribly. I'm looking into cutting some good closed cell poly foam to insert into the well. Haven't found any as yet.
The mag release is designed not to be set off accidently so it's recessed and narrow. At first i was very worried that i was going to have to put a pursuader on it to use it with gloved hands. After fixing the spring problem it became quite easy to operate and not much of a hassle. It's not as comfortable as your rifles release button but it doesn't need to be. It will only engage with the tip or side of your thumb so some larger handed folks might not like this so much. I just run my thumb along it back to front until it catches and the mag drops.
I would recommend using this ready mag system for carrying pick ups or empties you don't have time to dump or a back up mag that you need readily available and super duper quick.
I also found that when using my Magpul P-mags clamped together, the mag well sat at the exact perfect flush height to allow me to use both clamped mags with no interference and the mag well actually qualified as a cap cover, so no need for those durned snap covers! It does mean the mag well has to be empty at the time though. It's cool and makes for good aesthetics, but unless you run your gun like a typewriter, inconsequential.
Metal is solid. Madbull solid!
Screws are strong and low-lowish profile
Flexes enough to adjust to your rail for proper fitting and alignment but rigid enough not to be bent
Allows you to carry an extra magazine at close hand.
Allows for the use of clamped magazines at the same time (sits at JUST the right height)
Does not hold magazines firmly (suggest a big block of high density closed cell sealed foam)
Release spring Might be too long to allow ease of use (snip of loops on the spring a quarter at a time till you get the right fit)
Mag release button might be a little painful at times
Ready mag systems are not for everyone or every gun (not a con, just a warning)
It's very pricey. THink good and hard about what you need it for. A riser or full top rail riser is one thing, and a ready mag is another that will work a little bit different and a bit better with standard/typical AR-15 type receivers.