I bought the bathroom fitting kit after doing extensive reading on my options for drilling holes for wastes etc into tiles. By looking after mine I've kept the diamond cores going for 6 months so far (I do about 1-2 bathrooms each month at the moment). I've only ever drilled into ceramic tiles with them but they work as good as they did the day I bought them.
The guide they come with to save pilot drilling also saves wear on masonry drill bits too. Overall, a really impressive product and much better value for money than more expensive solutions.