We tried one. Incredible PITA. Very much not good results. Pig ended up way too much like it had been steamed-braised, without that underground pit goodness. Uses a TON of coal. It's just pure indirect heat like an indoor oven, and doesn't get any smoke to the meat at all.
Only thing to recommend it is that you can go outside and drink beer while the better halves are plotting amongst themselves. Otherwise... nothing good. Don't even think of using it for anything smaller than a whole hog either. Too much trouble -- and you can do much better cooking in a variety of other ways.
What you're left with is something that sort of mimics an underground pit, but not all that well. It's probably worthwhile if you and your family do a couple of pigs a year, you have a tradition of underground pit cooking (not as smoky and moister than open or closed pit) and no one has enough backyard to dig an actual pit. In that and only in that case -- a definite maybe.
My 2.l07 cents.
Rich