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I went through a period where I used a Muse almost daily, and it's definitely fun/useful when you're starting out, but I found the positive feedback (birds) only kicks in when you have a particular type of focus on your breath. There are both false positives (you may be distracted and thinking while keeping a superficial focus on the breath and the device will report that you're totally focussed) and false negatives (you are well-focussed on both body sensations and breath but the device is reporting that you are distracted).

So it works for a particular type of meditation, if you're relaxed in a particular way, if the battery level and contacts are good, if the initial calibration went well. It's great for starting/restarting a meditation habit but I would say one outgrows it fairly quickly.

Edit: the following pay-what-you-want course really helped me; it even improved my Muse scores! I have no affiliation.

http://www.peterrussell.com/HMWET/index.php