The Mach 6 is Hoka's light, quick trainer. No plate and no gimmicks, just a fast-feeling foam and a low 5 mm drop in a shoe that weighs about what a racing flat used to.
Use it for the days with structure in them. Tempo runs, cruise intervals, a half marathon you want to run hard: it holds a pace without asking much of you, and it stays honest about what you are actually doing.
It is not a max-cushion daily trainer, and it is not a super shoe either. There is less foam under you than in a Clifton, so back-to-back long easy runs start to feel firm, and there is no carbon plate to flatter your last few miles. Think of it as your second pair rather than your only one.