I do it, but I do other things along with it. Can't say it alone gave me results, but it's one of the safest exercises to build the core without putting much stress on your back.
Yes its great exercise for the core and for cardio. You can also work the obliques and hips by bringing the knees up as far forward as possible , one leg at a time.
@Sweetpoison If its just when you're doing planks then it's probably because you've not really ever worked that area out before like me. After doing planks for a while I could go longer without feeling tense as quickly.