Personally - don't waste your money. If your cats or your dog decide that the squirrel running by looks like something it wants to chase - it is probably going to blow off that zap on the way out of your yard. That same zap will probably prevent them from returning to your porch since there really isn't anything interesting or quick moving to distract them from that zap. Besides that - an invisible fence isn't going to keep anything from coming into your yard to get said pet.
Your better bet would be to put a traditional fence around the backyard - or part of it - and teach your pets that if they must go out, the back door is acceptable.