Much like insurance, it doesn't seem like a big deal until you need it. Recently an equipment failure during the transport/installation of a rather large piece caused a major chip and the piece needed to be repaired.
First step is to fill the hole. I make sure to mix up a VERY large batch, about 2-3 Gallons worth. The closer that you can recreate the conditions of the original pour, the closer the color match will be.
It gets covered with plastic to help prevent shrinkage and cracking.
A good bit of grinding and polishing reveals the sand and blends the color.
Pinholes get filled in just like the original piece and then the finish goes on. Complete, it blends almost perfectly. You can tell but only if you really know what to look for. Otherwise it just seems to be a natural variation of the piece. Not as easy with granite or marble.