There isn't a go-to item for her. You either see the mage-assassin build (DFG rush) or the normal mage build (grail/deathcap). With the DFG, she can delete someone. With the grail/deathcap, she's a little more survivable and can still delete someone (although much more difficult to do), but has a lot more teamfight presence.
I personally think she sucks. She doesn't scale well. She is about as fragile as Sona. Her cooldowns are incredibly prohibitive to actually dueling someone you can't instagib. She really requires you to win early or rely on a hypercarry ADC. She's the Renekton of mid lane without anywhere near the lane dominance.