We fly on Southwest, so we get free luggage. For that reason, I was never able to make the math argument of having an Owner's Locker make sense. I held out for a long time based on that.
After hearing so many people raving about it, though, I finally just gave in and decided to give it a shot. Once we had our first locker, I realized it wasn't so much about savings as it was about convenience. I don't have to organize, pack, and lug that extra suitcase(s) of stuff through the airport any more. When we pack, we just pack our clothes and the few items that need to be replenished. We know everything else we'll need is already organized and will be waiting for us when we arrive.
Also to the convenience argument is that your locker will hold things that you'd never think to pack in a suitcase to take on vacation, but sure come in handy when you need them. For example, we have a nice pair of scissors, some real kitchen knives (DVC's are pitiful!), a Swiss Army knife, a small tool set, some zip ties, and a roll of duct tape in ours.
In the end, there have been some savings. I can buy bulk size products like shampoo and shower gel (the products we want to use, not just whatever's being used by the resort that year). We can have good coffee in the room, and not have to buy cups of it each morning. Also, we use Sweet-n-Low, which is hard to find at WDW. We have several bottles of alcohol, and can mix ourselves a drink in the room which is certainly a savings over Disney bar prices. (I never would have packed something like that before our locker.) A big savings for us is having the Brita pitcher, and not having to buy so much bottled water. We also don't have to throw away unused quantities of as many things as we used to at the end of a stay.
After time, we graduated to a second locker. We typically do several short studio stays a year and one longer trip that will be a week in a studio by ourselves followed by a week in a two-bedroom with friends or family. Our lockers are organized along these lines. We have one locker with "every trip" essentials, such as the laptop, power strips for phone and camera chargers, umbrellas and ponchos, a Brita pitcher, coffee supplies, a bottle of rum, the scissors, a laundry bag and laundry products, a Scrabble board, night lights, first aid supplies, and limited quantities of a broad array of toiletries and medicines. Our second locker is the "extended stay" locker that has additional games, pool toys, BBQ utensils, spices and kitchen supplies, the knife set, bulk quantities of toiletries, several other varieties of booze, and a folding cane in case my knees start acting up on longer trips.
When I got a new laptop a few years ago, the old one became my WDW laptop and stays in our locker. Now I don't have to mess with that extra step at the TSA screening. All I carry now is a small USB hard drive that holds the photos I took during the trip.