Just seen guardsman get it on lux. Mana, movement speed, ap, and a sheen proc.....but when is it necessary? I just never seem to want to do double damage as an AP caster.

Any ideas?

lichbane. Late after you have decent ap. Endgameish. 6th item. + tower damage.

I usually get it when the enemy has alot of Ranged. If they have 2 melee DPS it's not really the best buy

Unless it's a core item (Akali, AP Twisted Fate), you won't see much use out of Lich Bane. It's generally acceptable to get it either as a 3rd or 4th item.

well usually if you have an ap caster with fairly short cool downs (around 4-8 seconds each) and a good amount of ap (usually 300+) its usually after a DC and its good at the end of the game when you have your DC and defensive items like RoA or rylai's or zhonya's and you just need an additional kick in your damage

Lich Bane Lux is not something I have ever done. I much prefer Nashor's, so I can actually make use of my passive, both for farming and for dealing damage in a fight. Also, 25% CDR while not losing out on AP is very very nice. Some people claim the AP is "wasted", since Lux is a nuker rather than an auto-attacker. But her passive is nothing to sneeze at.

I find it's generally useful for hybrids (Jax especially), but that's about it.

You saw it on a Lux because of her passive, which makes her really the only AP champ worthwhile getting it. Now that I think about it though, perhaps it could be viable on Vlad too?

Once saw a Ryze buy one, I lol'd.

Get it on rapid casting assassins (akali, poppy) or as a final item mages with good range on their auto attacks (anivia, annie).

As long as you have over 200 AP the Item is more effective then most people give it credit. It's a free 1 AP ratio as physical damage on your auto attack every 3-5 seconds.