With any product there are 2 ways to answer the question: Use value, as in how the product works and exchange valve, or how is the materials and workmanship.
Use value of a big green egg or any kamado barbecue is without a doubt the best outdoor cooker because the product and radiate internal high-temperature heat like a pizza oven beyond the capabilities of many gas barbecues, it can barbecue at very low temperatures and everywhere in-between. No level of heat is off-limits with a kamado.
Where this is extremely useful is at low temperatures where heavy ceramic and upper/lower vents that make up the kamado design allow the chef to hold low temperatures for a very long time with almost no adjustments. The charcoal heat, smoke and wood smoke are kept locked in the large egg shaoed barbecue so food barbecues slowly and absorbs the flavors of the charcoal smoke, wood smoke and vaporized drippings.
We barbecue and we smoke on our kamado. We make turkeys every Thanksgiving, whole beer-can chickens, amazing(!!!) ribs, bacon, steaks, hamburgers, hot dogs, fish and steak. The kamado can have the vents set for a specific temperature and it will cook all day left alone. Open up the vents and it jumps up to 350, 450 or even 900 degrees. The kamado is the most versatile barbecue, smoker, grill available and it is perfect for cooking anything.
The Big Green Egg has moved fabrication operations out of north America but the ceramic still has a life time warranty. Stainless steel parts have a life time warranty. Other metal parts are covered for 5 years. The Big Green Egg, their Distributors and us, the dealers make it very very easy to use the warranty to replace parts. There are not a lot of questionnaires and pictures to email but in 12 years in the barbecue industry I can count BGE warranty calls on one hand. They are just really well made.
I also have a Primo kamado and a Grill Dome and both of those are also really well made with similarly easy to use long warranty periods; lifetime on the Primo.
With a reliable brand the kamado is the best cooker available -- and like the nautilus and the orchid it has not needed to change to evolve in a very long time because its simplicity is its perfection.