Downside: more javascript (another file to load, to break, to parse, to wait on, etc)
Downside: unusual behaviour. Users fully understand how to scroll to the top of a page to find the header. This hack already requires them to scroll up to get to the header. Also, there's a decent chance that if you're trying to find the header that you're going to leave the page, so it doesn't matter if you lose your spot.
It's a cutesy feature, but it's cumbersome and pointless, and doesn't add any real usability (unless you count getting rid of fixed headers, which also don't add any real usability).
Downside: unusual behaviour. Users fully understand how to scroll to the top of a page to find the header. This hack already requires them to scroll up to get to the header. Also, there's a decent chance that if you're trying to find the header that you're going to leave the page, so it doesn't matter if you lose your spot.
It's a cutesy feature, but it's cumbersome and pointless, and doesn't add any real usability (unless you count getting rid of fixed headers, which also don't add any real usability).