A personal computer in 1975 cost about $2,500 in today's dollars if you bought it assembled. It didn't do much, as there was no third-party software and it only had 256 bytes of memory. (However, for an extra $1,350 in today's dollars you could upgrade it with another 1,024 bytes of memory). A few thousand units sold that year, which was a shocking success.
As a laughably downsized version of what were then giant industrial machines, it was hard to see then what a personal computer would eventually be useful for. It would be another four years before Visicalc--arguably the first really useful thing you could do on a personal computer.
So would you have bought a personal computer in 1975?
In retrospect, many people would probably say Yes, but that's only because we now know how the technology evolved. It's much harder to be in 1975 and see how this expensive toy (which is all an Altair 8800 would ever be) would change the world.
It's this line of thinking which made me decide to buy a home 3D printer. I want to learn about this new technology not because it's useful today, but because it has such interesting potential over then next 10-20 years.
Commerical grade 3D printers are powerful pieces of industrial equipment, and have no place or purpose in the home. Their smaller cousins have only been on the market a couple years and are pretty much expensive toys with limited practical value.
On the other hand, a small 3D printer isn't much more mechanically complicated than an inkjet printer (in some ways it is actually simpler). There's no reason that, with enough manufacturing volume, someone couldn't sell a 3D printer for only a few hundred dollars and put one in every home.
The only reason nobody's selling millions of cheap 3D printers is because nobody knows what your average household would do with one.
I don't know if we will discover the Visicalc of 3D printers, the killer app which transforms this from expensive toy into useful tool. I don't think a lot of people in 1975 knew what the future held, either.
There was another interesting piece of industrial technology scaled down for home use which came around only a few years after the Altair 8800. Unlike the personal computer, however, not too many people today have a personal robot.