I used the sticker on my 57g. It actually works and it does get more sensitive with time passing. I bought it only b/c when i get from work i need to see at once what i need to do. When there is no amm. it is very bright happy yellow, if there is 0.25 and more it gets "dirty" yellow. If it's greenish it means energency overdose of amm.
Unfortunately it do not measure nitrites. When cycle goes next stage you can be happy with your amm. but all your fish can die from huge nitrite surge. So, it is expensive and it won't save you if you are not careful.
When i did my cycling, i first looked at the sticker, then did my amm. and nit. tests. And then did huge wc. Why? B/c if you have like 0.5 of nitrite you need to do at least 80% to bring it down to "safe" level and hope for the best.
So, the cycling is not for faint of the heart. At the end it supposed to be worth it but it depends on your feelings