On a serious note, agility drills are great. They're fun, but surprisingly demanding. They could work really well for a high intensity cardio workout. However, I prefer to perform them at the start of a conditioning workout (after a good warm-up) and have a fair rest between drills. They have a decent degree of difficulty and risk of injury if performed with poor form, while fatigued.
I've got the book containing the complete Nebraska football program. Its an amazing resource for any athlete who requires speed and agility.