I have one of the UnderArmor shirts that I got from LApolicegear.com. They have their uses, but it's not a every-circumstance kind of undershirt/holster. I find that it can be uncomfortable unless you carry a lightweight gun, like a 442, because it seems to pull unevenly on your shoulders.
I've worn mine several times when it was necessary to wear a suit for formal occasions but wearing a belt holster would have been inappropriate should I need to take my jacket off. I put my 442 inside the weak side pocket and a couple of speed strips on the other side.