I can only share my own experience but it can certainly happen. Last year at this time I was 150 lbs (goal). From then until May, I gained 26 lbs back (of the 120 I had lost). This was no fault of the band -- it was me eating candy, chocolate, cake more often than on a "treat" basis. It can certainly be done - but it has to be on the effort of the banded person (in MY opinion) to do so.
If you follow a healthy "diet" (and for me, again personally only, that means about 1600 calories), exercise on a consistent basis and have "treats" only as "treats" and not as a daily part of life, you will keep the weight off.
I have about 16 lbs now to get back to goal.