Kalista

She is fine.... people tend to play her wrong and ultimately randomly. First off max the excute on the Spears. If you think she is weak look at varus and at how weak he is now and look at kog....

She is slated for some work, seeing as her kit is problematic. I'd say pick up a different marksman, as we don't know what Riot has in store for her.

She's in nowhere near as bad a state as people say. If you're duo queuing with a support and have some type of voice chat it is extremely easy to win early game and carry up until very late game. Best kiting, free smite, long range wards for scouting objectives, fantastic initiation tool with her ultimate.

That being said however... Vayne is better than Kalista in every single way with the exception of utility. She deals more damage, can kite almost as well (doesn't really matter since she can 3 shot anyways) and really just basically invalidates Kalista. Kalista also has an absolutely horrible late game that is really only better than Kogmaw. If your support doesn't understand how Kalista functions or you are camped by the enemy jungler early game, you're completely screwed. You cannot 1v1 any other adc at any point in the game unless you are an item or two ahead as you are completely reliant on your support. The only people you'll be 1v1'ing are Kogmaw and possible Varus and Caitlyn, provided you dodge their skill shots.

Really Kalista isn't bad, there's just a lot of better options. So much of your damage and utility is loaded into your support and even with your support you still have a horrid late game. If you want to pick up Kalista I would say it's not worth it. It would be much better to pick up Jinx, Cait, or Vayne. Kalista does however have some changes coming up around 6.24 or early season 7 so maybe it will be worth it at that point.