Where its at its best is on turn one helping you cast a ton of mana rocks, your Tymna, or a Rhystic Study, or slightly later, maybe on turn three, where it helps you double or triple spell some of your most impactful (but more expensive) cards
If you're only casting one half of a fuse spell, you declare which half when you put it on the stack
Not even the obscene amount of fusions can make Dark Magician good, and thats even with a couple of really good ones such as The Dark Magicians and Dark Magician The Dragon Knight
In any set that includes them, they make up the most expensive cards that you can pull
Excluding the "Hieratic Seal" Main Deck monsters, the "Hieratic" Xyz monsters, and "Hieratic Dragon" of "Sutekh", "Asar", and "Nuit", all "Hieratic" monsters trigger a mandatory Trigger Effect when they are tributed (regardless if from the hand or field), to Special Summon 1 Dragon monster from the player's hand, Deck, or Graveyard, who will have 0 ATK/DEF
You ignore any timing restrictions based on the card type of the card you copied