I work my legs often, 3-4 times a week, but use short workouts. I like to just use the lower body portion from MIS, Power Hour and CIA 2102 Powerbar Training. The Firm's Lower Body Split is excellent, but long - it has both floor and standing work. I also like Gilad's Quick Fit Hips, Buns and Thighs - he doesn't use weights for most of the workout, but I use my barbell and that makes it quite challenging.
When you're ready to spend the money, though, CTX is a great series - Leaner Legs is my favorite (well, toughest) leg workout and the other strength workout is very, very good as well.
Erica