I have a circa 1999 Mark XV. For at least the past ten years it has not run very well. It sort of runs out of power after less than 24 hours, and runs slow.
I have had it serviced multiple times by IWC. It has only ever been serviced by IWC. They fix it and then it comes back and works ... better, for a while, but still seems to struggle to stay powered for 24 hours. It works when I'm very active, but when it's just a regular day at the office, often it runs out of power.
Again, it's been serviced by IWC multiple times. I've spent thousands on servicing it. I'm getting a bit tired of it and thinking of selling for "parts / non-working" on Ebay.