I know someone who is on them, he's lost a ton of weight but they're now hooked on them as they're worried about what would happen if they did come off. He wasn't particuarly fat, quote a tall broad guy, maybe a couple of stone overweight but looked okay as he's a big guy. He looks gaunt and ill now.
If you've got a huge amount of weight to lose and are at risk of further issues then i can see why people would take them but for the person that is capable of exercise and diet it just seems a lazy way of doing it.