Does anyone buy death cap anymore?

Sometimes it just depends on your champion and what your against. Though for someone like its pretty much mandatory on him due to how well it synergizes with his passive Phenomenal Evil Power. If you thought he had lots of AP just from poking his laming opponent constantly wait until you seem him with 1000+ AP due to the Passive on .

Lol I didn't even know it was nerfed I always buy it still. What would be a good item to replace it with on karthus akali or ahri

It's more of the fact games are ending before the 4th item (usually deathcap on mages). only Veigar seems to be rushing it or getting it second so he's seeing it a lot.

it's worth if you need to stack AP and have a few items as the passive "scales" in gold value

This item is one of the most powerful items you can possibly buy. It's a late game item and it costs nearly 4k gold though, you don't build it until very late into the game.