I have always worn CK undershirts, and after reading about these in an article got some.  They are obviously really well-made, but they are VERY form fitting - like suck every tiny curve.  It was never my intention to wear these and nothing else, so it's not an appearance issue (although it would appear better if I appeared better :-P )  It's that on the hottest days, I found that it made me feel extra hot, almost claustrophobic -- it could be that I am just not used to wearing clothes that tight, but if you are like me, you might find it too much.

I am interested to see if the form fitting thing feels like an advantage when it's otherwise colder out.  Also, and this is minor, but with my other shirts and a dress shirt, the amount of tucked-in-ness was always a shared thing with the two, but with this it's almost as if the undershirt is on it's own level, and the shirt can be tucked more or less separately - which takes some getting used to.  Hope this helps someone decide :)