For the ones new enough to get an SSD upgrade, it's night and day the difference (even a Power Mac G4 can feel fresh and fast just swapping out the drive). For older Macs like PowerBooks and classic Macs, there are so many SD/CF card to IDE/SCSI/etc. adapters now, they also get a significant boost.
But part of the nostalgia of sitting there listening to the rumble of the little hard drive is gone.