While I can see where they would be occasionally useful, mostly for making replacements for small, hand-to-find parts, I'm not ready to buy yet. I thought about it last summer but for the money, I got an oxy-actylene set up instead. I figure in 10 or even 20 years, the gas welding setup will be worth about what I paid for it whereas the 3d printer will be worth approximately nothing, like a 10 year old computer.
I'm not trying to say that these are equivalent goods with equivalent capabilities, just that I will get more use for more years out of a welder.