They are a big thing, especially in Texas where they and abt's come from. The yankee version (according to Bigwheel a.k.a. fanatical Texan) are the most popular. The true Texas way is to stuff a plug of yellow cheese (colby) in the chile and wrapped with sausage.
Back to the yankee versions. Italian sausage and mozz is good, as well as any seasoned form of ground meat. A ground beef and pork mixture with sage, cumin, cilantro, garlic, onion, and chile powder also works well.