Maybe I'm not sophisticated enough to use just the tall-tales, but I prefer to have both those and the Windex.
You're plenty sophisticated if you realize that you need both. Tell tales tell you if you're trimmed properly. The windex tells you whether you're headed or lifted. So when the tell tales go flat, you glance at the windex and know why. Several seconds later the knot meter will catch up and show you slowing. A minute after that
, the GPS will finally get around to telling you that you're slow.
Tell tales and windex provide the most bang for the buck of any performance instrument on the boat. By far.
BTW - Tell Tales on the mainsail's leech are about the only way to tell if the main is stalled or trimmed correctly. People often assume that the jib is the only sail that gets tell tales.