You spend much less money when you have to fork over the cash. It is way too easy to spend money you don't have, or money that could be used in a better way, when you use your debit card.
When Casey and I first switched to this cash system, it was really, really hard for the first month, but it got much more fluid quickly. I felt a tremendous sense of relief knowing exactly how much money we had and what I could spend. Your use of the credit card will go way, way down. Let's say I was buying a wedding gift (BCS, before cash system), I would panic wondering last minute if we really had enough in the checking account to cover the cost, so I would charge the gift.
Today Casey and I have no credit card debt. We haven't carried a balance in years. The key- don't spend what you don't have. You really have to refuse to spend money. Don't borrow from those other envelopes!!! You will not have the cash you need for those license plate tags when you need them!