Unless I'm staying in a DVC unit, I always stay concierge anymore.
This is why:
I like the private check-in. Nothing sucks more than spending five or more hours traveling, to get to the hotel and have to stand in line for another half hour or more to check in. With concierge you simply tell the guard at the gate or the valet/bellman, or both that you are concierge, and they'll take you immediately to the lounge or your room (this varies by resort.) If the room isn't ready they takeyou to the lounge where you can relax and have something to drink or nibble on, and sit in a comfy chair while you check-in. If the room is ready they take you right to it, and when you're ready you can go to the lounge to meet the staff and check-in.
The service is personalized. They know your names and greet you as you come into the lounge. They remember you from visit to visit, they know what you like and dislike. Some of them keep track of your birthdays and anniversaries and you'll recieve either a card or phone call at home on those days. If you celebrate an occaision while styaing with them you'll get a cake and card or bottle of champagne and a card as appropriate.
The rooms are often in better condition then some of the rooms on the non-concierge floors, and often have some of the best views. The concierge floor is a private floor or area with keyed access. It keep the transience and noise level in the hallways to a minimum.
There are bathrobes in the rooms, which saves on packing. There are also upgraded toiletries and room amenities, such as free in-room fridges. Also automatic turn-down service each nice. It's so nice to come back to a freshened room. They do more than just turn-down the bed. They replace used towels, take out the trash, close the drapes and leave a soft light on. It's very welcoming.
Of course there is the lounge. Snacks, appetizers, cordials, soda, breakfast, wine, in some cases bottled water you can take to the parks with you. It's also a nice place to relax in a "homey" atmosphere where you can mingle with other guests, watch a little tv, etc. Some of the lounges have fireworks views.
There are other perks, which vary by resort. Free stroller and wheelchair loans, the Sunrise Safari, the ability in some cases to score hard to get PS's, the best seats at restaurants, etc.
Having someone take care of all your room requests and PS's in advance is wonderful. If you do it via e-mail they will send back a list of all the PS's and the confirmation numbers so you can verify if you wish. I've never had them mess up anything on me. It's also nice to have them take care of things like floral arrangements, gift baskets, etc. for you.
There are so many other reasons to stay concierge. But if you are looking for value you can quantify with actual hard costs, it won't happen. Much of the cost is for the higer level of service, which is harder to place a value on, as it's subjective in nature. For people who are willing to pay for pampering, then it's something you should look into. If you are as the OP says "Frugal", it's probably not going to be "worth it" to you. To each his own. It's definitely not for everyone.
Anne
Whatever that 'je ne sais quoi' is, I believe that Concierge will provide a good deal more of it. 
