I don't do this very often, and I don't often get clients calling who need to do this, but I just got a call from someone who has a small office who's switching over from HP PCs to Macs.
I have conventionally used an external backup drive to do this sort of data transfer, where I copy the material from the PC to the external drive and then hook the external drive to the Mac and copy back in. For large quantities of data this was faster than getting everyone on the same wireless network and copying over the air (wired was not an option, which I know would be much faster, but I still don't know if that's faster than the external hard drive method).
However, I also would probably have the option to pop out the PC hard drives (at least if they're laptops) and use USB to hard drive interface cables to treat those drives as though they were external drives. I have never tried this on a Mac, though I've done it many times on PCs.
I'm just curious about what the cohort has to say about what are the better/best/most efficient ways to do mass data transfer from old PCs on to new Macs.