The Macho Brace works similarly to Super Training - if you look at the Super Training graphs for your team members, you'll notice they've already been trained even if you haven't actually used Super Training yet. This is because KOing Pokémon gives you effort points, just like Super Training. The Macho Brace (and Pokérus) doubles effort point gain, so the graphs will fill out faster, but you're still limited by the gauge on the right - without the Macho Brace, this will fill only half as fast, but it will still max out eventually.
Even for competitive training, it's better to use the Power items (Power Bracer etc); although they only raise one stat, they're way more efficient than the Macho Brace.
