Is it worth buying schematics?

Anecdotally, there were definitely stretches of the game where the best item for a particular character was something I could craft rather than something I had found. If an item's level is looking low, it might be worth messing with, particularly if you're enjoying the crafting.

TL;DR Yes, it is.

Basically, unlike crafting in a lot of other games, a schematic isn't for a specific item, but for a category of items, and only specifies whether a slot on it requires metal, leather or cloth -- and how many units. So as you find better materials, you'll be able to craft better gear from the same schematic.

Of course, better schematics use more bits in each slot and result in even better gear. As usual, check the wiki for a full summation and breakdown.