I found this HX-550 device to be uncomfortable, difficult to fit in/over my ear and difficult to turn on/off due to poor design.

The over-the-ear mounts didn't work well for me at all.  The smaller in-the-ear mount worked better.

However, I originally thought that the on/off control via the flip stem would be a good idea.  Unfortunately, in use it did not work very well for me at all.  Making the flip action unfold from the inside of the device (the part that is against your face/ear) was not a good choice.  There is minimal clearance when you are wearing the device.  So my big fingers with minimal nails could not easily find the end and open the stem w/o pulling the device out of my ear.

WHY put the flip action from the inside out, rather than the more logical from the outside down?  Or make the stem rotate from the top of the device down to fully deployed.  Either would make more sense!

During use, if I wanted to open this small flip stem to take/make a call w/o having the device come out of my ear, I had to use two hands - one to wiggle my fat finger between the device and my face to search for the stem end (up near the inside middle of the device) and the other hand to hold the device in my ear so it wasn't pulled out when I folded the stem out.  Using two hands to enable the device is obviously not practicable when driving.

As for the sound, my friends said I sounded OK and background noise was eliminated nicely.  But everyone sounded a bit muffled to me, as if the high end frequencies were being dampened too much..

Why don't these devices have some sort of tuning ability to allow one to determine the overall frequency response characteristics?

I would recommend caution when looking at this or any similar devices that have an on/off/microphone stem that flips open/closed to the inside of the device (against your face).
I found this HX-550 device to be uncomfortable, difficult to fit in/over my ear and difficult to turn on/off due to poor design. The over-the-ear mounts didn't work well for me at all. The smaller in-the-ear mount worked better. However, I originally thought that the on/off control via the flip stem would be a good idea. Unfortunately, in use it did not work very well for me at all. Making the flip action unfold from the inside of the device (the part that is against your face/ear) was not a good choice. There is minimal clearance when you are wearing the device. So my big fingers with minimal nails could not easily find the end and open the stem w/o pulling the device out of my ear. WHY put the flip action from the inside out, rather than the more logical from the outside down? Or make the stem rotate from the top of the device down to fully deployed. Either would make more sense! During use, if I wanted to open this small flip stem to take/make a call w/o having the device come out of my ear, I had to use two hands - one to wiggle my fat finger between the device and my face to search for the stem end (up near the inside middle of the device) and the other hand to hold the device in my ear so it wasn't pulled out when I folded the stem out. Using two hands to enable the device is obviously not practicable when driving. As for the sound, my friends said I sounded OK and background noise was eliminated nicely. But everyone sounded a bit muffled to me, as if the high end frequencies were being dampened too much.. Why don't these devices have some sort of tuning ability to allow one to determine the overall frequency response characteristics? I would recommend caution when looking at this or any similar devices that have an on/off/microphone stem that flips open/closed to the inside of the device (against your face).