Is it worth the investment? I've already put a good chunk of money into him and he's yielded some OK stuff for me.

You should max all the crafters (Blacksmith, Jeweler, Mystic). It's pretty cheap to do so in the grand scheme of things and they are insanely useful. Blacksmith is actually the least useful of the bunch as you won't really use him for anything but smashing stuff once you hit 70 but it really helps with levelling an alt character because you can always just make level appropriate gear if you aren't getting any for a slot.

Exactly. When, or should I say if, you start a new character, the blacksmith will be leveled up to the point where you should be able to make level-appropriate gear for them that will be more or less as good as what you'll come across. Eventually, you'll start finding better and better stuff in the wild than you can make, but by then the blacksmith will have served his purpose. Even then, there are some pretty cool legendary plans you can find that let him make pretty cool legendary items (like Reaper's Wraps).

The jeweler and the mystic are both a lot more useful in the long run, so definitely get them up as quickly as you can.