Also probably worth dividing PPC into the 32-bit architectures (601, 603, G3, G4) and 64-bit architectures (G5), although the PowerPC architecture did that transition in such a civilized manner that it wasn't really traumatic. In some ways the jump from the 601 (Power-ish) to 603 (true PowerPC) was bigger than the jump from the G4 to the G5.

They also never fully went 64-bit on PPC, so there was never really a transition there, just some 64-bit machines that could do some slightly different things and I don't think there was ever a full 64-bit version of OS X for PPC. So there weren't any huge painful breaks.

Good points. Worth noting that PPC -> x86 was a 64 bit to 32 bit transition.

I assume we will soon see ARMv8 -> ARMv9, but that will hardly be noticeable.

I owned the first MacBook Pro but it died before the architecture became an issue :(

Overall Apple should be good at architecture transitions by now as they've done enough of them!

Also the jump from 68000 and 68010 to 68020-030-040 had a similar transition (if anyone remembers having to worry about wether your software/system was 32 bit clean)