According to Adam Schefter of ESPN.com:
From @mortreport and me: Andy Reid has reached agreement to become next HC of Chiefs, pending review of contract by each side's attorneys.— Adam Schefter (@AdamSchefter) January 4, 2013
This is not surprising, because Reid was among the most appealing coaches available, and the Chiefs needed a more proven head coach than their past couple leading men.
Having led the Philadelphia Eagles to five NFC Championship Games during his tenure, Reid now takes over a Kansas City team that only qualified for the postseason three times in that same time span. Finishing 2012 with an abysmal 2-14 record, the Chiefs don't have anywhere to go but up.
As for Reid, consider Kansas City a great starting point for a fresh start. There's no pressure, and the rebuilding process can now begin.
The end result of what to expect for the 2013 season gives way to best- and worst-case scenarios for Reid come next fall.