I've heard from several people who have used them with success, and a couple who had faulty disks. Things to note would be that they only measure TOXIC ammonoia, most test kits measure total ammonia. Toxic ammonia is normally only a very small percentage of the total but its the bit that you really have to worry about.
I wouldn't rely on it as a replacement to normal water tests, but it does offer peace of mind.
thanks for getting back to me. i think i'll leave it in and keep up the normal testing of the water. i've got a lot of water changes ahead of me to get my water free of the locked ammonia so it'll be interesting as well as useful to run as many tests as possible.
but you're right, avoid too good to be true products hahahaha.
If you've been using an ammonia locking type product then these ammonia alerts are more useful. As they only tell you the levels of harmful ammonia and can ignore all the ammonia that is safely locked away. Regular test kits can't do this.