This is a great shirt for concealed carry under a dress/button-up shirt.  It fits tight, close the skin like under armor products.  It breathes well.  I'll know more when the weather heats up.  But-the shirt is great when you can't wear other modes of holsters for concealed carry.  You can't quickly access your weapon, you'll have to unbutton your over shirt or pull it up by the bottom seam to get at the carry shirt's velcro pocket.  This is meant for situations where you'll have time to duck and cover, assess the situation and pull your carry weapon.  Although a full-sized pistol may fit, a compact would be better.  And best of all a sub-compact fits perfectly.  I carry a Sig P238 and it fits and rides perfectly.
The concealed carry shirt isn't perfect for every occasion but every holster has a purpose.  You need more than one way to carry, can't just rely on an ankle holster or inside the waistband holster.  This is another great option!  I doubt you'll regret it.