It is great for when your pregnant or when your weight changes and your cup size doesnt. It is worth getting. I brought one when I was pregnant at the time and it was helpful and worth every penny. Hope this helps.
M&S do a pack of 3 (white, nude & black) for £5. I'm on steroids at the moment and all my bras are digging in really badly so I'm looking forward to being able to breathe again! Also, I have a couple of nice La Senza bras which are really tight in the band but were too loose in the size up. I thought, being cheap bras they would stretch quickly but they are just too tight to wait that long!