I worked at ITA Software full-time (Less than a year)
Great location and technology - world-class infrastructure
Long hours; heavy volume of work
The process took 6 weeks. I interviewed at ITA Software (Cambridge, MA) in October 2011.
Included four hours of grilling by engineers, a two hour programming exam, and lunch with an engineering manager. The questions were quite difficult, and there's no breaks in the process. You are being grilled for seven hours straight.