Me and my wife only took the first two miles of the trail to Hanakapi"ai Beach, where we took another trail to the Hanakap"ai falls. The part of the Kalalau Trail the we walked was a little strenuous but worth it for the amazing views. If you are in Kauai I would say this is a must see. Loved the time I spent on the trail and it was one of the highlights of my trip.
I will say that if you are going further on the Kalalau trail that it does get extremely dangerous. There are spots such as " crawlers ledge" where you are basically hanging off a cliff thousands of miles in the air. There was no such dangers during the first two miles and unless you are an adrenaline junky I suggest you stop there.