Warmer than you would think, but works.
I have 3 of these shirts (from Botach Tactical, $35/each). I can honestly say they are a disappointment. I had been carrying OWB all winter and decided with the warmer weather I would try IWB and this... They itch (the stiching scratches) until they're broken in and are a lot warmer than they need to be.
I can hide my wallet in it easily and could carry a passport in the other side. It's a wonderful way to carry something you don't want pickpocketed. As for a CCW, the problems are 1) it feels conspicuous (not unlike a belly band, but less comfortable) 2) the shoulder straps need a balanced load and 3) you must unbutton your shirt to get to your CCW.
I can't carry my M&Pc9 in it at all (print through everything but a heavy sweatshirt).
I have found that if I carry my PM9 in one side and two mags in the other, it's balanced enough, conceals well and I can carry under a tucked-in dress shirt. I can wear slacks that fit and with or without a jacket, it's okay.
The advantages are obvious- it conceals small CCWs well, it's not on your waist (where skinny guys don't hide anything well) and there is nothing outward (especially if you wear a tie to cover the neckline) suggesting you're carrying. You don't need to disarm to ride in a car or sit at your desk.
I don't want to sound too negative. I like it- it works for this business casual guy. I can wear a sportcoat and can take it off without showing. (If anyone is sensitive to TMI, skip the next sentence) One other point is you can visit the facility without laying your CCW on the floor with your belt. A Keltec p32 disappears (but is a little difficult to grab if you need to) and a PM9 hides very well. With the right overclothes, you can hide anything. I think these are a good option if you are limited to a dress shirt and slacks (with a tie, or get the V-neck if not). A good quality tuckable IWB is probably a more reasonable option unless you simply can't carry IWB.
js