WotA is very useful with ability power-based champions that have health costs on their spells (e.g. Mordekaiser), practically resulting in a negation of ability costs.
Cooldown or Energy-based champions such as Kennen and Rumble can also make use of WotA for the sustain boost.
The spell vamp aura is also a major boost to any allied attack damage-based casters and assassins who deal their damage primarily through their spells, but cannot efficiently gear for spell vamp themselves.
Note that its prerequisite item, Hextech Revolver, will already provide the holder with a reasonable amount of spell vamp. The upgrade into a WotA does not increase the sustain effect very much, and thus should not be a priority until your team will greatly benefit from the aura.
Note that the aura will be lost on champions who become untargetable upon spell activation, making this a useless item for an AP Master Yi as the AP+Spell Vamp from the aura won't be available when he uses his only damaging spell, Alpha Strike and that the aura won't be available for the allies of any champion that goes untargetable, such as Fizz.
The owner of the item will receive both the 50 ability power and the 30 ability power from the aura for a total of 80 ability power overall.
If you and a teammate both own WotA, your aura will stack with your ally's aura, adding up for a total of 110 ability power and 40% spell vamp for each champion possessing WotA as well as being within another WotA aura. This buff only stacks once.