I have had a pretty dreadful experience with missing baggage with KLM for 4 days of a 7 day holiday! We were flying ABZ-MUC via AMS.
If they could just tell you the "truth" at the outset, it would save so much heartache. Initially they told us it would be on the next flight. Then told us it would be on the first flight the next day, then it was the day after. Then we found out our bags were at MUC but they wouldn't courier them to us (2.5 hours away from MUC) until the next day.
They told us we could track our lost luggage via a weblink. This turned out as much use as a chocolate fireguard. So lots of grief, expensive phone calls, different story each time you called. Terrible.
On a slightly different note a good few years ago I used to run a Bed & Breakfast on an beautiful Island of the West Coast of Scotland. I had regular guests about 6 times a year from a German operator who brought walking parties to the Island. I can honestly say that 50% of the time some poor holidaymaker arrived without their baggage. It did seem to arrive the next day from Glasgow so would involve a long courier trip from Glasgow to Oban, then a ferry from Oban which was not cheap in those days around £60 return, then the drive back to Glasgow. Must have cost the airlines a fortune in getting these bags to pax.