Menendez Mags are very reliable, as far as round feeding reliability is concerned. People that use them claim that they follow the open slide action of the firearm well, ejecting and loading a new round in the chamber with ease.  Some have reported the slide locking too early, but with light sanding on the leg of the follower, that problem is easily resolved. Leaving a magazine loaded may also tend to swell the body, so it may have a harder time sliding into the magwell; that won’t be a problem if the magazine is stored and unloaded.