Akali loses a lot of survivability as soon as the enemy gets Oracle's/Vision Wards. That doesn't make her worthless, but she's a lot harder to play then.
Akali can burst very well and especially not-so-good players underestimate her damage output. Due to her passive (spell vamp) and her resource system (energy), she can lane very well. As soon as she reaches level 6, her ganking potential is awesome.
All in all, yes, Akali is cool, but she can be countered in several ways, which requires you to be very good.
I don't have LeBlanc, so I cannot say much about her.