I know this thread is over a decade old but in case others come across it doing research as I did, cajunsmoker's comment "And, under no circumstances should you grill or smoke over woods such as cottonwood, willow, pine, or poplar." is not relevant to magnolias.
Please note that Sweet Magnolia and it's cousin Tulip Poplar (aka Yellow Poplar or Tuliptree) are NOT part of the cottonwood, willow, and poplar family. Poplar/Aspen/Cottonwood (which are mostly all names for closely related trees of the
Populus genus, as well as willow, are in the Salicaceae family. Sweet Magnolia (
Magnolia virginiana) and Tulip Poplar (
Liriodendron tulipifera) are in the unrelated Magnoliaceae family of trees, so information on cottonwood, willow, or poplar (and pine) is irrelevant, which isn't to say that Magnolias are necessarily any better for smoking.
Also, for reference, the updated link that the quote comes from is:
http://www.fieryfoodscentral.com/2008/07/08/mesquite-and-other-woods-for-grilling-and-smoking/
It does not mention magnolias.
Tuliptree wood burns to a white ash not a coal, so it may not work well for smoking but should be fine for some kinds of cooking and I use it frequently because they tend to fall in my yard :-\ . Some sources I've seen suggest that its smoke adds a somewhat sharp spicy flavor, I've noticed a somewhat sharp smell when cooking.