Hey,
some time ago I heard that Sejuani was kinda useless...
As she is free now i immediately picked her on my first game and took the jungle route from blue to wolves to wraiths to golems to red and I was kinda surprised.
My first impression was like :"Oh my god, she has to go back after wraiths or at least after golems" but then I realised that her ganks are very powerful and that her leveling speed is still ok. You stay in range with your level even if you gank a lot. And I really mean a lot.
I just followed a guide from here that was voted the best and went 10/5/27 or something like that after my first game Sejuani!

To draw a conclusion I would say that Sejuani is not useless especially not in teamfights. Her AoE-stun can completely shut down the enemy team and if u charge in with Arctic Assault while having Northern Winds activated u deal tons of damage and your Permafrost slows the enemy team so much that they can't get away from the teamfight.

I would like to hear your opinion about Sejuani and her strength :)

Thanks to Rezpite for his excellent guide .