We got the QS dining plan last year when they had
free dining. For reference, it was just my fiance and I and we are both in our mid 20s with healthy appetites. He prides himself on being able to pack away a ton of food, and with the QS plan we definitely struggled to use all of our meal and snack credits. I believe we left the World with 1 or 2 unused meal credits, and we had a surplus of about 8 snack credits [we stayed 7 nights] that we used right before we left at the caramel store in Epcot Germany to buy brownies for our family members back home.
So regarding the amount of food with QS - there is PLENTY of it. I feel like at Disney the food just builds and builds in your system over the course of the vacation, so by the end we were actually almost sick of food. Keep in mind, too, that with QS you get twice as many snack credits as you do with the regular dining plan. The snack variety you have to choose from is really immense -- and the snacks can be VERY filling. Snack credits are great in Epcot with all of the international offerings to choose from. They are pretty much like small meals all by themselves, and are a lot of fun IMO.
In regards to the variety - I don't think I had a single hamburger. My fiance did, but he just likes hamburgers. We stayed at PoP and ate at their food court several times over the duration of the trip. If you travel to other resorts, you can really get a broader variety. We were able to get Tonga Toast at the QS dining area in the Poly

There is plenty of variety in the parks, as well; especially in Epcot. Definitely look at the menus for the QS dining areas and think about the places you will want to try. Having more snack credits means more variety, too. We also shared our snacks and meals with each other so we could try even more things!
With the money we saved by not upgrading, it more than covered the cost of us eating at a table service restaurant or 2 out of pocket in addition to what we had with our QS meal plan. We are usually only interested in eating table service at 1 or 2 places during our trip, so this works out well for us. I am not interested in having to make dining reservations daily or plan around the dining. I enjoy the freedom the QS plan gives me.
If a person travels to Disney rarely, maybe having all of the table service credits would be more attractive since they may want to squeeze in as many of the big name table service places they are interested in as possible; however, if a person travels to Disney every year or every couple of years, I don't see the need to cram them all in at once and spacing them out by trying one or 2 out of pocket might be a good idea.
So having had the QS free dining, I asked my fiance what he thought for the future and we both agreed that we will keep the QS.
In summary: We really like all of the snack credits, the food was good, variety was good, the freedom was good, and the savings was good.
P.S. - I realize this is my first post so I don't have an established reputation yet. I'll be honest, I have been a lurker on this forum for quite some time as more of an information seeker than sharer. This post caught my attention, though, so I registered so I could contribute.