My dad was going to get a Science of Cambridge Mk14, but Sinclair's famous customer service meant that it was late and he cancelled the order and instead built a ZX80 from a kit. I learned to program by reading the manual in bed at 11 years old. I am not sure my life would have gone the same way if I had had to learn the Mk14!
The problem with the ZX80 was the flickering/black screen - it took CPU away from maintaining the display every time it ran a program, so moving graphics were impossible in normal programs (I believe using machine code you could do something but my memory is dim now).
That was the big advance in software with the ZX81, it had "slow mode" which allowed both the display and your own logic to be run at the same time (what luxury!).
The great thing about the ZX80 kit was that it came with a full circuit diagram, and that apart from the CPU and memory it was all made from discrete standard logic chips, so you could get an understanding how it worked at a hardware level.
The ZX81 replaced all those logic chips with one custom device, thereby rendering it a literal black box to the likes of curious youngsters like me.
I progressed to a Spectrum and later via fellow Z80-brethren Amstrad CPC to "proper" computers. But the ZX was where it all started, as for many.