What grinder do you recommend for the NEO Flex?

The type/quality of the grinder needed depends on which portafilter you'd like to use!

The NEO Flex arrives with two portafilters: The Flow Control and the Bottomless 2-in-1.

Thanks to the  Flow Control Portafilter, you can craft espresso without use of a high-quality burr grinder, as any lack of fineness/consistency in the coffee bed is supplemented by its pressurized properties. Any grinder will do, so long as the grind size can be set coarse enough to avoid clogs/build-up. You can even use pre-ground! 

However, if you're ready to take total control of your extraction and brew with the "non-pressurized" Bottomless 2-in-1 portafilter, you will need to use a high-quality, espresso-capable grinder paired with fresh beans. This is necessary in order to slow the flow of water through the coffee bed to generate the needed resistance/back pressure for genuine espresso brewing.