Mine were replaced at 67k km (42k miles). I was getting a repeat CEL and it was diagnosed as misfires. The car was just off warranty by about one month. The dealer spoke with BMW Canada and they covered 70% of the cost (which included replacing the plugs) as a "good will". My SA also cited that the plug replacement was called by CBS and that the manual was incorrect. The CBS never showed that the plugs needed replacing though. They replaced them as part of the troubleshooting for the misfire.
The car was better with the new plugs. I had the software updated under warranty a few weeks before, so the software and plug combination made a good difference. The car really felt as if it was pulling more, more responsive too.