Just saw/participated for the first time a few weeks ago. Made a whole lamb, birria style, Mexican flavors - banana & avocado leaves, epazote, guajillo chiles, etc. Great results, but uses a lot of charcoal - if we had done it the entire way in the caja china, probably at least 60-70 lbs of brickettes. Ended up taking it out, breaking it down a bit and finished it in an oven. No smoke flavor since it's sealed in the box. Cool and relatively inexpensive way to cook a whole animal, though.
I have used one, with mixed success. The difficult is keeping the temperature of your fire relatively low, especially if you have a large piece of meat inside. If it sticks up too high and your fire is too hot, it will burn on the top. And the design of the box is not really that you should be frequently manipulating the meat.
It does use a lot of charcoal, but actually uses less compared to an open-spit roast or similar. You can get some chips smoldering and throw them inside the box for smoke flavor.
I own one and have for about 8 years. Its easy to use and good fun....I hav exclusively done pigs and the results are nice usually goes as far as tender meat is conrned. My biggest issue with it is getting the pork skin crispy in the box,it will never be close to when you cook a pig on spit...I remove the skin before carving it up you can cook it further on my grill or in a high heat oven.the only I have figured out how to get nice crackling