Build Quality

Overall build quality is very solid. There is no rattle when shaking the mouse. When applying lateral pressure, no creaking or flexing of the shell can be observed. Activating the side buttons by squeezing the sides is impossible. Lastly, accidental clicks when slamming down the mouse do occur, albeit only if debounce time is set to 1 ms.

Buttons

Main buttons on the ES21M are good. Pre-travel is moderate and post-travel high, resulting in a fairly firm and snappy button response nonetheless. Being visually separated from the shell, button movement is moderate when provoked. Button stiffness is medium to light. A pair of Huano (green transparent shell, white plunger) switches are used.


Side buttons are excellent. Pre and post-travel are kept at a minimum, and actuation is accordingly pleasing. The actuation point is even across the entirety of these. Button size and placement are good, as actuation is possible rather easily by rolling one's thumb across. A set of surface-mounted switches from Huano (white plunger) are used for these.

At the bottom of the mouse is a button for cycling through the set CPI levels, which works just fine. A seemingly unbranded tactile switch is used for this one. A slider at the bottom switches between on and off-state, which also works fine.

Scroll Wheel

The scroll wheel is good to very good. Noise levels are mostly under control, and tactility is quite good, with mostly distinct steps allowing for controlled scrolling. The encoder comes from TTC (blue, yellow core) and has a height of 7 mm. The middle (scroll wheel) click requires high force for actuation. A seemingly unbranded tactile switch (black plunger) is used for this one.

Surface

The ES21M has a matte surface all over. Grip is fine, though it does attract fingerprints and dirt to a significant degree. It is mostly easy to clean, but getting it fully clean is difficult if not impossible. All in all, decent materials.

Button Sound Test

Disassembly

Disassembling the ES21M is easy. The screws are located beneath the rear skate. After removing that one, several clips arranged along the edge of the bottom shell need to be dislodged, doing which is most easily done by wedging a credit card or something similar in.

The internal design is surprisingly complex. A plastic assembly is erected above the main PCB, which holds the MCU and sensor, is affixed to the bottom with three screws, and connects to the battery placed towards the rear. A secondary and largely perforated PCB holding the scroll wheel encoder and main button switches is affixed to the plastic assembly with four screws, which in turn is fixated to the bottom with another four screws. The side-button PCB is clipped to the plastic assembly, and an FPC is used to connect the side-button PCB, main-button PCB, and an LED to the main PCB. The MCU is a Nordic nRF52840, whose datasheet is found here. The Bluetooth capability of the chip remains unused.


As for the soldering and general quality of the PCB, I'm unable to find any noteworthy flaws.