The MX W1 headphones are a set of wireless in-ears from Sennheiser that are currently available for about $500. As a general rule, wireless technology means poor audio quality (the audio has to be compressed for wireless transferral); in-ear headphones also tend to have worse audio quality than full-sized headphones. While the stars are aligned for the MX W1s to have horrible audio quality, they actually didn't perform that poorly on our audio tests. They were far from perfect, as we'll describe in a bit, but they were far better than what we were expecting. 
 

The benefit of not having to deal with a cord wiring your ears to your front pocket (or wherever you're keeping your media player) is certainly nice, but you end up partially paying for it with comfort. To get these headphones to fit, you have to insert them, then twist. This braces the headphones against your ear canal and the inside edge of your pinna. As you can imagine, having a bit of plastic wedged into your ear isn't terribly comfortable. Further, the way they fit into your ear is akin to most packaged-with-device headphones, namely they sit by the opening of your ear canal but don't actually block it off. This lack of isolation might make the MX W1s slightly less than ideal for commuters, who often listen to music on noisy busses or trains. Further, the headphones themselves actually take up more space than traditional in-ear headphones, since their ear buds are comparatively huge and the transmission device will add bulk to your media player. 

One last thing to consider are the aesthetics. These are headphones you should try on in front of a mirror before purchasing. The MX W1s essentially look like a pair of Bluetooth headsets. If you like this look and can pull it off, then congratulations: you are one of the lucky few.

Performance  (read the full lab performance results at HeadphoneInfo.com)
As previously alluded to, wireless technology and the in-ear form factor doesn't bode well for performance. The MX W1s, however, managed to not have abysmal audio quality. They have a surprisingly good bass response for a set of headphones that don't create a good seal with the ear. Their main issue is a weak high-end. 

They had far more distortion towards the low end, but this distortion didn't extend past the bass frequencies. Oddly, given their relatively high distortion levels, the MX W1s were still capable of outputting an impressive 117dB without significant distortion. Typically increasing the volume will worsen distortion significantly. 

As we also mentioned earlier, these headphones don't isolate well at all. In fact, these headphones isolate worse that just about any headphones we've reviewed. Again, this is because the headphones only partially block your ear as opposed to sealing it off entirely. The MX W1s are like holding a full-sized speaker close to your head as opposed to the ear plug-like effect of other in-ears. Regardless, the MX W1s had above average leakage control. While we wouldn't recommend listening to loud music in a library, you won't be deafening others in the room. 

Comparisons  (read more in-depth comparisons at HeadphoneInfo.com)
It's hard to compare the MX W1s to other headphones we've reviewed since they're the first set of wireless in-ears we've had in. Really, if you're considering these headphones you need to ask yourself a few questions. First, do you care about audio quality? If yes, then you should give these a pass. Secondly, are you on a budget. If yes, then you should definitely give these a pass. While the value of wireless connectivity is entirely subjective, we're not entirely sure it's worth the asking price. This being said, we do hope Sennheiser keeps working on the technology, with the hopes that some day it becomes more comfortable and affordable.