Boozle can be a great tool. Unfortunately it relies on input to keep the prices up to date. If a store changes the price, and no change is made on the site, you can end up driving all the way to what you think is the cheapest, only to find out it is not.
I work in a BWS outlet, and see many customers come in and quote Boozle prices. Depending on the store, they may agree to match the price quoted, if you can show them evidence of the price on your phone. However don't expect it every time, and my own experiences are not necessarily reflective of my employers. Some BWS stores may choose to match, others may not.
I have been a Boozle customer myself and I have found it very effective if I have been out at a friends or relatives and need a top up.
When updated regularly, it is a great tool. Just be prepared to be caught out occasionally and carry that extra couple of dollars.