I built one last year and it's great. It took a couple of cooks to fine tune it, but now that I know its quirks, it works really well. They're very efficient and the coals can last 14-16+ hours (long enough to smoke pork shoulder and then some).
The key to maintaining temp (225-250) is to make sure all your seams are sealed so there are no air leaks. Air leaks were the biggest problem right after I first built mine.