Doubles any effort values the Pokémon holding it gets, but halves its Speed in battle. Stacks with Pokérus.-Bulbepedia
It depends really, if your Pokemon really relies on speed to win a battle, then I would say it's not worth it. But a Macho Brace only lowers speed in a battle, so if your Pokemon has other ways to win a battle without speed, Go For It!
Hope This Helped :D