While nothing has happened to make my MacBook Pro 100% useless, the optical drive became disabled in such a way that it would not burn dual layer discs, twice, and the battery issued service warnings, twice.
However, I am notoriously rough on my hardware, and carried it around every day for the past few years, so I tend to be the exception rather than the rule.
However, despite the dents and scratches, Apple still replaced my DVD±RW/DL drives and batteries, and in less than five days.