I've been using the Seachem kit. At first I thought it wasn't working but once I tested with the reference sample I learned even though I dosed my tank, it was all absorbed the first day or so.
Once small tip of you decide to go with this kit, you only need a few grains of power of the dry reagent. It doesn't come with much and if you use too much, you won't get too many tests from the kit (it advertises 75 tests). I spoke to Seachem and they confirmed only a few grains are needed.
I can do the test now in just a few minutes. Once I add the grains, I walk away for a few minutes then come back for the reading.