To me the Waimea Canyon is what makes Kauai special and different most almost every island anywhere on Earth. The Canyon is beautiful and if you are going to be in Kauai I feel that it's a MUST see! I'm shocked that the US hasn't made it into a National Park and started to charge a fee. I loved that this was a NO COST adventure.
We were here a week and kept watching the weather report so we could attempt to time it when it wasn't raining. We got lucky! It was hazy out, but at least we didn't get wet and still had some amazing views. If you happen to be here at a time when it's not cloudy, raining, overcast, or hazy you are VERY lucky! The mountain range to to East gets over 450 inches of rain a year. The most anywhere on Earth.
I will agree with some the drive is longer than the amount of time you will spend looking at the Canyon, but if you don't really enjoy the first stop just turn around and go back to town. Otherwise I would recommend driving all the way to the end. Every stop offered something different to see with the very last stop being of the coast.
I recommend taking hwy 550 and not 552. It's a smaller road but the views of the valley are amazing!