I wouldn't ever take diet pills. I wouldn't even take caffeine. Many of these diet pills are filled with caffeine and chemicals. If anything I think drinking acai juice (known to boost metabolism and immune system, and detoxify) , wheatgrass juice (detoxifying and known to help lose weight) , chili powder(metabolism booster because of the heat) ,and many other harmless foods would be a better way to go.
Diet pills can be addicting and dangerous, and they cause some pretty crazy side effects, including the most disgusting ones (WARNING):
anal leakage, and loss of bowel control (Must I add more?) The most common diet pills aid you at losing weight by making it impossible to digest more then about 25% of fat in a meal. If you eat more then 15g of fat in your meal though, you'll poop your pants. Even if you don't eat over 15g of fat in your meal, you'll still have grease pouring out occasionally... 
Don't know about you, but I'd rather be fat.