I've used one for several years. First one died after 18 months or so, Wahoo replaced it, second one has been solid. Replaced a Scosche Rhythm that died twice and I elected not to try a third one.
IME the optical arm bands are a little more finicky to get accurate readings out of than the old chest straps. They work, but seem to take longer to pick up changes in heart rate or even give an initial reading. But they are waaaaaaaay more convenient to use. I find I get better results on my right arm than my left (who knows why). Just below my elbow on the inside of my arm seems to work best. You may have to play with positioning to figure where it gets the best reading.