Speaking as a gymnast i can tell you it takes ALOT of work to be able to do full leg raises, to the point where your legs are vertical, and parallel to your chest.
The flexibility is more of an achievement than the strength required to do them. However, once you have devloped that flexibility. You have a great full abdominal exerices.
The real problem is with these, that most of the show boaters go to the chin bar and start doing leg raises, trying to touch their feet to the bar.
The succseed in doing this, but you look at their form, and you realize that they havent gotten any farther past 90 degrees than they would have had they done them regularily. most people get their legs vertical, by leaning back and making their torso horizontal.
This is a riddiculos waste of effort. Ask your partner how your form is, you should make sure that you are not doing what a described above.
If your looking for the flexibility to do this. I reccomend alot of hamstring streching and back bridges.
With this sort of flexibility you can pull of some incredible and very effective abdominal exercises. Actually most of the best abdominal exercises only become avalible once you have achieved the flexibilty necessary to allow for this full range of motion.