The one reason I tell people not to go on crash diets (losing more than 5 pounds a week) is the fact that your skin will look like loose flab after you lose the weight. Your skin needs time to adjust to not being stretched over your fat. Unless you want to pay for cosmetic surgery to remove all that extra skin, do yourself a favor and don't go on any crash diets. You will find that even a wrinkle cream that works will not remove that turkey neck if you lose weight too fast.
By losing weight slowly you skin will have a chance to bounce back to a slimmer you.