redneck1st: I wish that I would have waited before I spent the crowns on him. But I wonder if I hadn't if I would have had access to the runes and such for which he offers? Because I'll have to admit that he does carry some great runes and schematics. As for adding slots so far the Ursine Heavy armor I'm wearing I couldn't add no slots to it.
I think he carries some unique items, so yeah, it's worth it to give him at least some money to set up his shop.
You can only add slots to items that have less than 3 slots already (the Witcher gear always has this).
Example: you have heavy armor with 1 upgrade slot. You can now ask the runewright to add two additional slots (for a good chunk of money, of course).
You can now let the runewright use these slots to enchant the armor (e.g. the "all armor is light" enchantment) or just use them like you normally would. It can come in handy, I think it's one of the more useful things he offers.