This subject has been brought up before but I have just cleaned the lights on mine and thought I would share the pics ...
I didn't do a before and after picture (just an after !)
I was given this t-cut by someone in work to try today so I gave it a go. My headlights were by no means badly stained but were starting to tarnish around the edges, particularly on the top of the lights, so I gave it a go. 5 minutes (if that) was spent on each light and the polish took the stains away - they didn't need a lot of time spent on them, quite a bit of pressure and it was done by hand. they have turned out very well but like I said, they were not badly stained.
It was tried on badly stained lights by the guy in work and it didn't work (Ford) but perfect on mine. I will see how long it lasts and might do it every so often just to keep on top of it.