They are quite effective but you may want o also try other methods to keep the cats way. One of the other alternative you can apply is planting things that discourage cats such as lemon thyme, lavender, garlic, geranium and other plants that are strongly scented. You can also use an automatic sprinklers that are installed with motion sensitive detectors. When the cats get to the lawn, the sprinkler will immediately go off and chase the cats away. You may need to move the sprinkler around every once in a while so that the cats do not learn how to subvert it. Dogs are also an effective way of chasing away cats and you may therefore consider having one around.