Having used Seachem ammonia Alert meters for years, I find them indispensable.
I've tried Mardel meters but have found them to be just awful. Inaccurate readings galore and you have to replace the cartridges every 4-6 weeks. They're also 2-3 times the cost of the meters by Seachem.
The Seachem meters don't replace proper testing of water parameters but then they're not meant to. They simply alert you to Ammonia spikes.
They won't give you a specific reading but rather let you know at a glance if there's a problem ranging from alert to toxic.
From that point you need to get out your test kits and confirm your ammonia level.
The meters perform exactly as intended - serious hobbyists have been relying on them for years as a tool to prevent potential disaster.
Seachem states each meter will last a year, but I replace mine every six months or so - to stay on the safe side.
At the minimal price for these meters I find them an exceptional value when used correctly for their intended purpose.
Seachem ammonia alert meters provide added peace of mind (and with these fish we need all of that we can get) - not to mention they warn you of potential disaster and loss of fish by ammonia poisoning.
The best price I've found is on Amazon where the Ammonia and PH meters cost $8.00 apiece.
It's money well spent.