I've had SoundMagic EM10, ES18, Cowon EM1, Sennheiser Cx180 etc.
Build Quality:
Cowon > Sennheiser > SoundMagic. The cables on soundmagic will split itself, especially the left one. It is not a typical rubber tube, but rather a sheet of rubber rolled and glued to form a tube. If you have a sweaty neck or face, it will happen sooner. It won't last a year of regular daily use. The cables will sometime become sticky and oily, like the rubber grips in old pens

Sennheiser earphones sometimes lose volume on one of the earphones, like after six months or so. Glad it had a two year full replacement guarantee; got it replaced twice. Cowon has withstood all kind of torture for more than a year, and yes, it is tangle free unlike the other two where you have to spend quite some time untangling the cables.
Sound Quality:
SoundMagic > Cowon = Sennheiser. If you love bass, Sennheiser is the one for you. After listening to soundmagics, the Cowon EM1 felt flat. But still it has got a good bass and clarity. Sennheiser CX180 also has got a similar feel like Cowon.
SoundMagic EM10 got a better bass and sound stage than ES18/ES19.