I've been maining support for a long time now and I obviously know when to choose spellthiefs over the other gold items for different champions but I never know when/if I should upgrade it into FQC. Are there any champs that it is actually good on?
poke champs
I like getting it on Lulu for that slow.
She's a kiting machine, give her another slow to go along with q and it's op.
Mage and poke oriented supports.
The problem is that Talisman's active surpasses it in pretty much every situation, AND has a shorter CD.
Good on supports with extra bullets coming from autos like Lulu or Bard. I really hate 
, and I worship 
, but 
/
/
is great for playing Bard or Lulu and also Morgana (which you know).
I like to think about how the game will play out, when I buy my first item. if I'm tank it's between relic shield and amulet coin, if I'm mage/poke it's between amulet coin and spellthief's edge. I guess ocasionally i consider spell thief on blitz, for the added damage, and one combo consumes all the stacks anyway.
if I think I will be passive in lane, I usually go coin. if I'm going to be active trying to poke or skirmish, I'll get the edge. if I'm looking to disengage or make picks, I'll get the frost queen claim. if I think we'll team fight or siege, I get amulet coin upgrade. if the enemy has a lot of staying power, like multiple enemies with dashes, or enemies that don't care about slows, I'll get the amulet coin upgrade.
the thing is, I've gotten kills from the frost queen, and I've found it's better at encouraging my teamates to focus. some times I give movement speed and people don't know what to do with it! but if I slow an enemy, people generally know to target it, or better yet, it's an enemy I've seen trying to flank and my allies haven't yet noticed, somehow I get a thrill out of knowing I've dream crushed someone and only me and them know it :D generally, if I think that the slow will be useful for disengage or pick potential, I always go that path. otherwise I go coin upgrade. oh and, in a very frantic fight, the movement speed usually exaggerates my team's disarray but the frost queen almost always exaggerates the enemy's disarray.
Feel free to comment if you think I've missed some.
I don't use it much, but I always thought it's slow missile speed was better for running away than to chase. If you lack a slow, like Morgana, upgrading might be worth it.
The only support I'm decent at.
mage supports or just harass oriented supports who can keep sending their abilities to proc its extra damage and who are too squishy to be focussed on.
i hate that item
No, even spellthiefs just sucks. The income it gives is too situational. is pretty much dominant, even if you don't finish upgrading it. Just get 
and go for a real AP item.
