If you're talking about the t-shirt type, it works VERY WELL !!
Holds the pistol tight against your torso, and is easily accessible !! You'll like it !!
I have three, first one is about 5 yrs old. They work great. I wear a tshirt with no sleeves under mine and size them smaller than what I normally wear. Keep it fairly tight and they snug a weapon very well. I have carried an xdsc in one side and a spare mag in the other, I've carried an LCR in there, most recently a DB9. You forget they are even there after a while, much better than a shoulder holster and a lot easier to live with than a belt holster. Worth the money imo.