I have two flasks; one 6oz (Jim Beam Black holiday package, last year,) and an 8oz. Both stainless, and well used. I agree with Chuck that higher proof bourbons are better for flask sipping. Being on the move, or whatever the situation is that necessitates a flask instead of your usual drinking glass, you're likely to be taking small or discreet nips, and stronger bourbon is more satisfying that way; it seems to "go further".
I have taken a flask to a bar on three occasions that I can remember, and it's not yet caused a problem. No need to hide it, but be discreet, and order at least one drink from the bar.