Leg raises work the iliopsoas muscle (hip flexors) which are designed to pull the knee upward.
Standing up, place your hand on your stomach and lift your leg out in front of you. You won't feel the abs doing much. The function of the abs is to bring the sternum toward the pelivs, not to raise the leg or knee. When you're doing various leg raises, you'll definitely notice your abs tense up. They're contracting as stabilizers to aid in the movement but are not working through a full range of motion.
If you're working abs you'll be dealing with crunching movements. You'll find plenty of people who swear by leg raises but I'd wager that had they never done leg raises that their midsections would be about the same.
If you want to add leg raises to your ab workouts, go for it, I just wouldn't make it one of my primary abdominal exercises since it's not necessarily an abs-specific exercise. Roman chairs, regular crunches and reverse crunches on an inline are all you need. You can add in some twisting motions to those exercises if you want. For obliques I do standing side twists with a light bar over behind my neck to hold onto. Also, try doing stomach vacuums after each set of abs for about 20 seconds.