I've only ever taken two devices back to Apple for repair. One was an ageing iMac that needed a new hard drive and the other was a MacBook that failed completely. In the first case, Apple replaced the hard drive at a price that was about 30% in excess of what I expected. The MacBook was fried and Apple replaced it completely with a new unit before the 12-month warranty period was up.
But a recent expose by CBC News in Canada has painted a less than stellar picture. Simple repair jobs are being over-charged with fixes that other repairers complete in minutes.