Personally I wouldn't bother using one until you can lie on your back, and with your legs completely straight, raise them up to 90 degrees then back down again at least 10 times whilst maintaining a completely neutral spine. A lot of people will do this but their back will arch so it's using all hip flexor and no abs simply because their abs are not strong enough.
In the exrx example the chick does a good job of keeping a neutral spine (though at the very bottom you can see her stomach "flutter" a bit which suggests she's arched a bit more)
Lying Straight Leg Raise
Because I have weak abs I get to about 45 degree and my back begins to arch significantly