I used these when I first set up my tank, as I didn't quite want to stretch to the £30 odd that the API Freshwater Master Test Kit would have cost me.
These aren't quite as accurate, as I have used a test strip, then taken a water sample to pets at home and the readings were different, and the nitrate/nitrite was showing higher on the API test than on these strips.
As long as the squares for nitrate/nitrite are the best they can be, I'm generally not worried. If they get to the orange zone (a handy colour chart on the side of the tube) then I tend to do a water change.
Would recommend for someone who has a well established tank that doesn't need frequent checking. Anything else, I would go with the liquid and test tubes offered by API, as you get a much more accurate reading.