... though I don't know if matrices respawn.
In the vanilla game they do not, and since they only have about a 10% chance to spawn in the appropriate containers it can be a real pain trying to acquire them. This also applies to Madness Matrices.
As a result, I ended up editing the loot lists to increase the number of containers the things can spawn in as well as setting said containers to respawn.
The enchantments they provide are not all that good, although they do increase the value of the item significantly so I usually save up the Amber or Madness Ore and whenever I do find a matrix I make an enchanted item to sell.