Poor for building strength, no. Poor for building muscle, to some degree. In functional terms, the abs work isometrically, so an isometric exercise is perfect and actually useful. I guess it does depend on the reason why you're exercising your abs, it's just that they're a postural muscle and it concerns me when people try to build them up without thinking about this. Many other exercises give your abs adequate work, squats and deadlifts performed correctly, along with most upper body free weight exercises.

If you must do isotonic exercises for the abs, move only from the neck and upper back, keep your lower spine fixed on the floor. Basically, do crunches rather than full situps, don't try to go all the way up, and make it a nice slow, controlled movement.