At the moment this is half a product.
The hardware itself is actually pretty decent. The dock is nicely constructed with high quality plastic, the game controller is a close approximation to the original Gameboy, and the cartridge reader does a nice job of pulling the game off of your 25+ year old Gameboy cartridges. That half is great. The other half of the product, where one actually plays the game, falls short.
The underlying issue here is that Hyperkin does not provide any software to actually play the game as part of their Smartboy app. The app dumps the cartridge data to the phone and then requires downloading a third party emulator to get the game up and running. The emulator does not account for the fact that half the screen is cut off by the Smartboy, so a number of features are not visible on screen - including the option to close the emulator so another game can be played.
Removing the cartridge immediately results in a red piracy warning that can't be dismissed unless the phone undocks first. Swapping out the cartridge and putting in a new one will dump a new cartridge rom but you will still first need to exit the emulator to load up the next game. It's cumbersome and feels very disjointed especially given how easy it was to swap games on the original gameboy.
Another issue is some tremendously bad input lag. I'm measuring well over 100ms of lag between the time a button is pushed and when something happens on screen. This is surprising given the controller is connecting directly via USB-C and not through bluetooth.
If Hyperkin can more tightly integrate the actual game playing into their cartridge dumping app I would be more favorable to the product. But right now it's something that feels very disjointed and aggravating to use.