Excuse me, Derek. Just wanted to reply to Malana.
As OG says, after a few years they start to look tired. It is possible to refurbish them; the surface that is. Using carsprayers' T-Cut or cutting compound can bring them back up again.
I paint them simply because I hate brilliant white. I've painted my own in Farrow & Ball. Surprisingly, it makes them look quite modern. I get a lot of good comments from people who just want to know what sort of windows they are. They're completely standard.
The principle reason for uPVC is durability. No rot, and the paint never peels or cracks. Also greater insulation from the 28mm glazing units. Better weatherproofing etc etc.
Across the road from me is a standard bungalow with white uPVC fascias; guttering; windows.............. and a porch-y thing that looks like an ice cream kiosk ............. hence the paint :o)