Don't know about cooking with it, but yes, eating it will cause spontaneous, uncontrolled vomiting (high school horticulture class, teacher told us this, one guy decides to test it. results as expected).
I would recommend against.
EDIT:I just did a wiki search. Apparently, the vomiting is a mental reaction, but the guy in high school sure did have a chat with Earl. I still say stay away.....http://en.wikipedia.org/wiki/Yaupon_Holly
Edit again: Once again, Wiki seems to have failed. USDA says it will make you puke....Darn those quality controllers.http://www.plants.usda.gov/plantguide/doc/cs_ilvo.doc