I used the Migration Assistant over Firewire over the summer to move my workstation from my MBP to my Mini, so I could put the MBP into service, and then migrate back again. It made the task incredibly easy, and I boasted to friend and family that Mac upgrades were smooth and trouble free.
Irony struck thrice in the months since.
First was when I had to (family obligations) upgrade a PPC 7200 and a G3 Bondi iMac to a shared G4 450MHz. Obviously, not a match for the 7200, and the iMac doesn't have firewire. Nightmare.
Next, I helped a friend's gf buy him an MBP (early 08) for his birthday, and handled the migration from his graphite G4. Unfortunately, that G4 didn't match the specs for Target Disk mode. Luckily I was able to pop the ATA drives into an external FW400 enclosure, and migrate that way.
Yesterday, I helped a friend migrate from a G4 running 10.3.9 (Firewire mode only), to a new MacBook. No direct mode would work; the external enclosure is FW only, and my LaCie FW/USB 2.0 enclosure was a cable mismatch to the old HD. To make matters worse, the G4 had a faulty power supply, and wouldn't boot after attempting a couple variations. What DID work, was to pop the G4's HD into the FW400 enclosure (mounted on my MBP), run Disk Utility to create an image directly from the old HD to the USB drive, mount the image on the new MB, and migrate from a local volume.
The trouble with Macs is that they last longer than the standard upgrade paths account for. Maybe that's why the build quality is diminished: to force people to upgrade within the compatible upgrade window.