iTunes is slow, bloated, cluttered, and generally maddening to search through. iTunes Instant, a super simple iTunes and App Store search cobbled together by a 15-year-old whiz kid, is the exact opposite. Which is to say it's a godsend.
Google Instant is lightning fast, no doubt, but Google was pretty quick to begin with, so it wasn't not too exciting of an update. iTunes, however, is slow like molasses. Or sloths. Or like a sloth eating molasses on a really hot day.