Can I get some feedback on Corrupting pot

Seems pretty useless to me. Okay of course this isn't true at all, but for 500G? Now while it gives you a lot for it's price and seems like a must buy on mana dependent top-laners, it never seems worth it just never seems worth it.
For 500G you can start an armor item with
for a measly 300G and still have 200G to spend on regular health pots, which heal you for more. The mana regen and the burn are obviously which pushes this pot to such a high price, but I still don't see it being worth it. Is it a noob trap? It was to me, but is there something I'm missing?