Is shadow and demonite stuff worth anything in hardmode, though?
It's like copper equivalent in a new world. It gives some defense, but you definitely want something more than this.
Honestly, Molten is closer to a hardmode copper-equivalent.
Don't completely knock Shadow/Demonite. Those are cash cattle (most profitable as far as I can see, by volume/difficulty:sales ratio) when it comes to netting an income.
Create tons of basic boss baits (worm food and eyes), and let them loose and quickly/easily defeat them, and you'll net a good 10+ gold per kill. Well worth it for the amount of time it takes to kill them with good equipment. And to make your sales more inventory-friendly, process the ores into bars; there's no net loss in doing that, and requires only basic tools (just a stone furnace really) to do that as well, just more room to fit more demonite to sell off.
And quickly gaining the materials to draw in the bosses (easiest being the EoW) is as simple as carrying tons of water candles and drinking a battle potion, and collecting like mad in the corruption for essentially 1 game day, and you should be able to fight EoW a good 5 times over (or 50 gold's worth of battles) with that much material.
It's the quickest way to buy safes, ammo (to improve), illegal gun parts, and etc.; I would also recommend always carrying a handful of piggy banks and an extra safe for any new worlds you make/visit, so you can plant some checkpoints here and there, as well as expand your inventory to double it's size, and if playing midcore, reduce losses while exploring.
EDIT:
Actually, to be most safe, especially midcore, carry your safe inside a piggy bank, as well as carry only 1 piggy bank on you at all times, while you have the rest of them stored inside the checkpoint banks as well as the home bank (leave one behind, snag another for next checkpoint. That way, if (midcore) you die, it won't be a total loss, and you can still recover what you stored within the safe and bank, and all that gold spent wouldn't be in vain. Just carry 1 piggy bank at all times and maybe 2 wood planks for it to sit on, unless you made plenty of them as you explored.
Also helps if you have wirecutters and such stored in them as well for when you find traps and such, or are hunting for resupply.
EDIT EDIT:
Has anyone made anything interesting with the wiring feature? So far, I've made a few simple security traps to keep my residents safe from zombies and rabbits. Simple pitfall drawbridge with dart traps set to a 1-second repeater. But using red and green torches, I made some nice little checkpoints for my hellivator so it's safe passage downward, unless I grapple a wall and flip a lever (and next to the lever is a piggy bank checkpoint), and then switch the lever back to keep the pipeline safe. I intend to make a fireblossom harvester later on with a magma pump system going through my hell garden.