You can read technical reviews elsewhere, as well as a detailed explanation of the sound signature. Being a relative new comer, I will cover more mundane characteristics: aesthetics, build quality, comfort, and where I found them the most useful.
I purchased the purple and blue version of the ZST. I'm drawn to bright, shiny things. Without the tips, the casual observer would be hard pressed to tell the are not custom IEMs. They particularly resemble UE CIEMs, though a tad less refined. The Cable is multicolored, though looks like muddled from a distance and feels rubbery.
The finish is flawless, glossy and transparent. The beautifully laid out electronics are on display, which is a plus in my book. Each earpiece is stamped with a gold, stylized ZST. These earphones look like they should cost many times their asking price. The only part of the whole product that looks $20 is the cable which is finished to the same high standards but feels rubbery and the multicolor pattern looks muddy.
For me, ZSTs are very comfortable despite not fitting well within my or ear canal. This is a problem I have with all IEMs due to my small ear canals. The provided tips are somewhat fluted and provide a comfortable fit and hold. Unfortunately, the units stick out from my head somewhat, so I cannot use them in bed. Despite this, I never felt uncomfortable or sore after wearing them for several hours.
Overall sound impressions
The ZST monitors are excellent for listening to vocal heavy songs. Singers, such as Elton John, Billy Joel, and Lionel Richie, all of whose voices dominate their songs, benefited from the ZST sound signature.
Songs where the instruments are just are important as the singer suffer, but only because the instruments are pushed back in favor of the vocals. To use an example, while listening to Queen, Brian May's guitar work is de-emphasized and Freddie Mercury's voice becomes even more prominent.
Pink Floyd has many songs where the vocals are almost "just another" instrument. Comfortably Numb, with it's soaring guitar solos , strong string harmonies, and percussion back beats are lost to the vocals. They still sound good, but I do not get chills down my spine during David Gilmour's solos
Then I changed the cable and everything changed. Sometime back I purchased an after market braided OCC cable, which I believe to be the same one used by the Magaosi X3 but with 2 pin connectors which I used to replace the rubbery thing provided with the ZST. And the music soared. Comfortably Numb once again sent chills down my spine.
Buy the ZST for $20 and invest in a $40 OCC cable. Yes, you are spending twice as much in the cable than the ZiEM, but for $60 you will end up with a pair of IEMs which, to my ears, sound better than, for example, $180 Etymotic ER3XRs. Definite thumbs up at the current price, but when paired with after marjet cables, these babies can compete with IEMs costing many times more.