This shirt works well under other clothes.  It is made of synthetic material almost like thick pantyhose or microfiber panties, and I would not wear it by itself as a result.  It hugs the body, so it makes a great undershirt - I'm using it under a white button-down and it disappears underneath while covering my bra perfectly with wide straps.  I put the vee neck in front and the round neck at the back for this purpose.

I normally wear small, but sized up to medium based on a review and I'm glad I did - it fits snugly.  While I'm sure a small would have stretched to fit me, the medium is not loose in any sense of the word.  I'm 32" around at the bust and 28" at the waist to give an idea of that sizing.