I have a cintiq 24HD the whole equipment is incredibly heavy and well built, in the beginning I was worried about the glass surface, now it's just like a drawing table The glass surface is treated for such kind of use, but take dust very easily.
I have a Cintiq 12WX. The surface that you draw on is a thick glass panel. Apparently it is some kind of composite with toughened glass, anti-glare film and whatever else. I'm pretty clumsy and my cat likes jumping on the Cintiq with all her claws. In over a year we haven't managed to damage or scratch it yet.
I have heard of people 'easily' scratching their Cintiq screens - but it seems to mostly apply to older models or people doing dumb stuff.