The article and comments show that as usual, the general public doesn't use metric prefixes effectively.

While it is technically correct to say "1.2 million km" or "1,200,000 km", it is needlessly verbose. It is written more succinctly as "1.2 Gm (gigametres)". However, it is incorrect to stack prefixes like "1.2 Mkm".

After I point this out, the usual complaints will surface: "But no one knows what a gigametre is! We're all used to talking about odometers in only kilometres. No one uses big prefixes." Oh really? Are you telling me you don't know the difference between a kilobyte and a gigabyte? Should we revert to calling a 2.4 GHz Wi-Fi frequency as "2.4 million kHz", because kilohertz is familiar to people working with audio frequencies and AM radio?

Overall, I think we should use the right prefixes for the right job. If you're talking about city blocks, use metres. If you're talking about a single trip, use kilometres. If you're talking about annual driving distance, use megametres. If you're bragging about how long your car has survived, use gigametres (or at least thousands of megametres).