I have had a granite worktop for nine years. They can scratch but only if youare negligent. My wife swears by it for jobs like pastry making, bread making, etc. but for the rougher jobs such as chopping veg. she always uses a chopping board. Even if you did manage to scratch it, I would think it possible, with the right materials, to polish out the lighter marks.
In my case the maker told me that in the event of a nasty mark he would tackle the repair himself. See if you can get a similar guarantee yourself before buying. Most
important, buy off a reputable maker and instataller. Do not go for the cheapest option!
As for keeping clean, after nine years,ours is still in prestine condition without problem.