FlightCaster (YC S09): United trounces Southwest in 2009 on-time %
blog.flightcaster.com
blog.flightcaster.com
Again, this sort of thing should be adjusted for routes and airports, but depending on your purpose in flying a route (going home, where your parents will pick you up? flying to LAX to catch an intercontinental flight with a tight connection?), a more granular breakdown is important.
Partner with Orbitz/Expedia/Whoever so I can see it while I shop ... it would definitely influence me to use that particular travel site.