The XL's layout is slightly different than the image above, but the principle is the same. Bottom vent into an air chamber below the fire grate. Burning charcoal draws air from air chamber (both through the fire grate and via the sidewall vents) up through the coals. Rate of burn is controlled by how much air you allow to flow from the bottom air chamber through the coals and out the top. Tiny openings in the bottom and top vent will get you a steady temperature as low as 175 degrees. Wide open vents and you can watch the temperature gauge wrap around itself (no ****...I opened it wide for a cleaning once, walked away for half an hour, came back to a temp gauge that only said 300...well...the needle had gone clear past the top of the scale at 750 degrees until it reached 300 again! I could barely stand next to the thing! And when I closed everything up again and it had cooled down, EVERYTHING inside it had turned to ash. Not a bit of grease anywhere. It was beautiful.)
You won't need to add charcoal mid-cook. I've gone as long as twenty hours with mine, starting with a full firebox of fresh lump mesquite, and when I've gotten done, I've still had LOTS of unburned charcoal in the firebox. And the other neat thing about it is that you can start it, get it to temp, sear a couple steaks for seven minutes or so, then shut down the vents. The fire goes out as soon as it goes through all the trapped air, and an hour later it's cool enough to put the cover back onto it...still with PLENTY of charcoal in the firebox!! So it's great for long smokes or short searing and everything in between. I've made main courses, side dishes, desserts, breakfasts. I've done thanksgiving turkey, prime rib, meatloaf, apple pies, macaroni and cheese, smoked apples, peach cobbler, pork loin. Anything you can think of, you can do with this thing (which is true of most grills, of course, if you know what you're doing).
There's a short learning curve to controlling the temps down low, but most of that is technique in building the fire to begin with (which only seems to matter with low-n-slow cooks).
You can expect to spend north of a grand on one of these things. DO NOT order it over the internet (BGE doesn't actually sell them over the net...so if you find someone selling them that way, beware). You might be able to locate one used on Craigs List and get a good deal, just make sure you go through it carefully and look for proper assembly and care. By the time I got done buying the XL, the nest (stand), the side tables, the place-setter (which is a heat shield for low-and-slow cooks...a MUST), and a cover, I was north of $1800. Let me tell you that I did my research before hand, read everything I could get my hands on about charcoal smokers, and talked to everyone I could find that owned these things (my pilot group at work has a big thread on the BGE on our message board now...heh). This was the best fit for me, and as I said, I have been 110% satisfied with everything about it! It will last me the rest of my life (so long as I don't tip it over), and I may even be able to pass it down to my kids.
If you decide to save up and go this route, you won't be sorry. I would definitely suggest going with the XL, though...because you can cook small on a big barbecue, but you can't cook big on a small one. Truth.