Some people here use them. Some say they're no good as they can take quite some time to change if ammonia appears and by then the damage is done, but I wouldn't necessarily consider this a bad thing. Most people don't test their tanks all the time and you're still going to pick it up more quickly than just waiting until the mood happens to take you to test your tank. Not a replacement for a liquid test kit, especially if you're cycling, but they might not be a bad thing to have when your tank is established. Whether they're worth the money though, that's another thing.