I have been buying these in multiples of three since discovering them a few months ago. They are tight around the pits, which is actually what I like about them now (it means they are actually unisex-like rather than 'men's') and they 'pull everything in' if you're a bit 'puffy' on top. The material is slightly coarse, but nonetheless comfortable. I have been wearing tee shirts under clothes all my life, but have been converted to these vests as they are so comfy. The only downside for me is the shrinkage in body length when washed. All my clothes are washed on a 30 degree wash and air dried not tumbled. These vests lose a few inches in length after they are washed. My wife has been stretching them out after washing and they aren't so bad. Still enough to not rise up over my belly though. Another inch on the length would take care of the shrinkage. That said, I'm going to get more shortly. Black and blue this time probably. The heather grey and white are nice looking and my favourite colours, but I want colours for all occasions to go under tops and shirts.
I wouldn't recommend sleeping in them unless you have small shoulders. Probably too restrictive for night wear.
For the record, I bough large and I'm 5'11", modestly sized with a 44in chest, 32in waist (with a fat tyre round the waist), and around 13 stone and these fit me perfectly and hold me in place for the most. ;-)