I dig a basket for camping trips. I'm just getting my Yak all spec'd out right now with home made bicycle mounts for our next trip - 4 weeks to Alaska.
The basket to me shines for a lot of reasons - it's fairly inexpensive, it's low profile, it holds a lot of stuff, you can use it to haul dirty gear or dirty / smelly stuff like firewood / gas cans without worrying about getting the interior of the truck dirty / smelly, it has a million tie down locations, etc.
I use it for stuff that we are only going to dip into once per day or possibly never - so bulky stuff like camp chairs, awning, tool and parts bag, gas cans, water jug, etc., goes up there. All that stuff would fit in the back of the truck but it just frees up room by having it on the roof. I've also previously used it to haul a second spare tire on a month long off road trip. It's also nice to have it up there for unexpected storage needs - like one time I laid a guy's bicycle up there that had broken a weld on the frame and gave it and him a ride into the next town. We also use the basket to toss stuff into when we are setting up camp - we take gear bags from the back of the truck and throw them up there to make more room in the back to sleep. It also is a nice base to mount an awning to.
It's easy to grab items off the roof by standing in a door sill or on a tire. I can load bicycles one handed and those are the most unwieldy things I put up there.
Downsides are wind noise, security, maybe MPG...? - never really noticed that since the MPGs suck no matter what. The baskets will rust if you leave them up there all the time and I personally hate the look of them, so I take it off when not in use. You also can't reasonably put something up there that is super heavy and / or that you will be getting into a lot daily, like a cooler, for example.
My Yakima Loadwarrior w/ extension (loaded pic is it being mocked up for our upcoming trip w/ bicycle fork mounts installed):
Little bit of extra wind noise but tolerable.
