$30 is just for the departure service, you can buy it on the spot if the lines look long, they will try to charge you more, but it's negotiable. It won't let you bypass the airline lines to check in luggage, which is usually the big bottleneck.
If you have a long wait once you're through security, the lounge is a nice place to spend time, the food is not very good though.
The arrival service is $50 more and can save you hours if the lines to get in are long, but it has to be purchased ahead of time. Many threads here, search is your friend.