I am always skeptical of after-market magazines for handguns. Many shooters, myself included, have experienced a lot of reliability issues with various after-market magazines for various handgun platforms. When Magpul entered the market of manufacturing Glock magazines I was interested as Magpul makes only quality gear, in my experience, and their PMAG for the AR15 is of course stellar. The Glock 9mm is my primary handgun platform so the Magpul magazines offer an alternative to the OEM mags. The initial attraction to the Magpul offerings is that they average half the price of the OEM option, but what about reliability, our top priority?
I bought a few of the 21 round Magpul GL9 magazines when that model came out and the reliability has been perfect. The 21 round mag is my carried spare for both my Glock 19 and Glock 26. It is longer, obviously, than even a G17 mag, but for a spare mag it does not matter. The other two attributes that make the Magpul magazines even better than OEM mags, in my opinion, are the lighter weight as they use no steel liner, and they are also easier to strip and clean as the base pad removes without the need to squeeze the magazine as is necessary with the OEM. On the heels of my excellent experience with the 21 rounder, I bought a couple of 12 round magazines that Magpul makes specifically for the Glock 26.
I extensively tested the 12 round magazine and the reliability has been flawless. I trust these as much as the OEM. The most obvious difference between the two is the Magpul magazine is slightly longer and provides two extra rounds. It offers a full grip on the Glock 26 which typically leaves the pinky dangling for most shooters with the standard 10 round magazine. This is essentially a feature that you will find excellent or useless, depending on your concealment needs. The Magpul magazine adds relatively little length for adding two more rounds, but the extra length does indeed make the gun a bit harder to conceal. I find that the 12 round Magpul magazine puts the size of the Glock 26 right between the Glock 26 with the standard mag and the Glock 19. It remains easier to conceal than the Glock 19 but it becomes harder to conceal than with the flush magazine. No surprise, obviously.
Therefore, I really like the Magpul Glock 26 magazine and I use it when I am dressed in a way that I can conceal it with no problems. If able to accommodate it the 12 round mag offers two additional rounds and more purchase on the gun to improve control, so all good. However, in the hot summer months when I wear no more than a light t-shirt I foresee sticking with the flush-fit magazine to provide that ever so slight reduction in height that does nevertheless make a surprising difference in concealment. The bottom line: if you carry a Glock 26 the Magpul 12 round magazine is excellent, though it does add some size to the tiny yet mighty G26, so use it when appropriate.