don't like them, don't have one. i hate this particular direction of fashion -- now i have trouble finding nice fitted tops in the shops! all I ever hear are that people are mistaken for being preggers when they wear them. a friend of mine is a model and wore one and people thought she was pregnant! your size doesn't matter, they make everyone look like they have big bellies.
and you know why i think they are in fashion? I saw lots of models wearing them, Heidi Klum for example. And you know why they were wearing them? Yes! They actually were pregnant. Lesson to be learnt - don't copy everything models wear, especially not what the pregnant ones wear (unless you're preggers yourself!)
my advice: if you plan on getting pregnant any time soon - go and buy loads of clothes now!
other wise don't.