I have traveled using this bra pocket several times.  For the most part I thought it was convenient.

First:  Yes it may show a little bulge under tight shirts but if you are doing the vacation thing you probably won't be too worried if your money is safe.

2.  I found it so convenient to pull my credit card and money out easily.

3.  BEWARE -  If you are walking in the heat and you sweat, yuck,  your money and pocket will be sweaty.  But.............  at least it's safe.

4.  Had a friend pick pocketed first day in Italy so.............  this is still better.

5.  Bought another pocket that was larger to go inside the neck and it was UNCOMFORTABLE having a string around my neck,  much preferred my bra pocket.