These are the best shirts I've ever bought. They are super thin (the white one is a bit see through when pressed against my skin) and super light, wick away moisture like nobody's business, and don't stink. I literally did light workouts in one of these for 5 straight days, without washing it, just to see if their anti-microbal/anti-stink claims were true. They are. It's pretty unbelievable.
I bought two of these, one white and one black, for a two week trip to central Europe. I didn't want to have to pack a bunch of shirts, so I packed two of these, and 3 polo shirts. I wore these as undershirts, and neither these nor the polos ever got smelly. I washed them in the sink after every couple of wearings, which was very easy to do. They dried completely in about three hours.
Not sure what more you could ask for in an undershirt. I'm buying more of these to work out in, and just for casual wear around town. They're expensive, but totally worth it.