Once you've paired the Echo and your running app, you can start a new workout, and the Echo shows you data while you run. The exact data it shows depends on the app — the Echo is merely a wireless display for whatever information your app decides to share. At the bare minimum, you'll see the elapsed time and the distance you've run; some apps also show your pace.
So instead of craning your neck to read an iPhone display strapped to your arm, or digging into your pocket to pull out your iPhone, or listening for the periodic audio cues that pretty much every running app is equipped to provide, you can just look at your wrist. But ... if hearing audio cues or looking at your phone directly are enough for you to stay informed, you really don't need the Echo.
The Echo has four buttons, which you can use, depending on your running app, to pause workouts or switch music tracks. In Strava, for example, as long as you're playing something stored on your phone (not from a streaming service), and you queue up your workout music inside the Strava app, you can use the top-right button to play/pause the tunes, and use the bottom-right button to record/pause your workout. (In case you stop to fix a broken shoelace, say, or buy a doughnut — no judgments here.) So that's handy too, but again, you could pause your workout on your phone, and control your music either on the phone or with the little inline remote that's (probably) on your headphones.
Once your workout is over, the Echo goes back to being a watch. It's not particularly attractive, nor is it hideously ugly. The Magellan Echo Utility lets you customize the basic display a bit by selecting the watch face and date style. It also shows your weekly average for the runs you took with it, which is a nice touch but nothing more.
The Echo works just fine at everything it tries to do. The problem is, who is it for? Serious athletes would probably prefer a real GPS watch with more features that don't rely on a phone. And casual runners probably don't care enough about seeing real-time run data on their wrists to shell out $150.
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