Mardels do NOT work, we have several and they never read right.
We also have Seachems (little round ones) and they work fast and are accurate most of the time, for us anyway.
They're not what you want to test your water with though, they're just a good general indicator to tell you if something is wrong with an already cycled tank.
If your tank is cycling you need to test every single day with a liquid test kit. They're good to have around anyway, because if the little indicator in your tank says the ammonia is high you're going to have to test with the liquid test kit and make sure if it's reading right.