There's not much to dislike about the Impi: it's faster to train, cheaper to maintain, and gets better flanking and XP bonuses than the Pikeman it replaces. Its low Production and Gold maintenance costs allow the Zulu player to field an army of Impi very quickly and without risking a bankruptcy.
But there's more: the Impi becomes available around the same time as Mercenaries (if civic and tech development have been keeping a similar pace, of course), which allows Shaka to form Corps! Large groups of Impi are very difficult for opponents to repel unless they have a comparable force of melee units (which have a +10 Combat Strength bonus against Impi), and if you've been keeping up with your civic development and manage to capture a few cities, you'll have Impi Corps and Armies at your disposal long before anyone else. Start training Impi en masse as soon as you unlock them, then send them forth to show the world the Zulus' might!
The powerful warrior regiments of the Zulu were already a formidable force when the ambitious Shaka taught them new techniques: combining the iklwa (a short, stabbing spear with a broader blade than a throwing spear) and the ishlangu (a large, oval shield with cowhide layered on top). They trained to form shield walls, both to blunt projectiles and to hide their true numbers, and attack with a “buffalo horns” formation—greener troops would sweep out and around to pin the enemy’s flanks (the horns), while a more experienced center would slam into the pinned enemy (the rest of the buffalo).
The impi training regimes under Shaka were nothing less than brutal, but harsh conditioning led to tightly bonded, well-disciplined soldiers able to carry out complex formations with ease. The impi became a highly mobile force, nigh-unstoppable against anything short of repeating rifles and high explosives.