The main benefit is that you can get some quick cash in a pinch.
The major downside is that the interest rates these places charge would make a loan shark blush. Yes the rates are THAT bad. The APR, annual percentage rate, in the state of Nevada for one of these legal loan sharks located here in Las Vegas is 430.18% with a checking account and 573.57% without a checking account!
Payday loans are a financial trap no one should ever get caught up in. If for some reason you can't pay off the loan on time you will be stuck making interest payments which add up rather fast. A person should never ever use these services except as a last resort in an emergency.