How is frozen heart on jax?

Frozen Heart solves Jax's mana issues and gives him more opportunities to use his skills. Randuin is a better armor item, though, since it gives more effective HP against physical damage (despite the lower armor) and the HP is also useful against magical damage. Also, the active is amazing.

I would say that Randuin is a better option unless you have issues managing mana with Jax, and/or the enemy team is mostly or full AD.

Randuin's Omen is typically the more effective item on Jax since it provides more health (which benefits the resistances on his ultimate) and a movement speed slow to help him stick to his target.

Frozen Heart is a very nice source of CDR, but it seems somewhat situation. Though, if the enemy team is AD heavy or their magic users aren't a threat, it can be a great pick up. Might be a good idea to get a Giant's Belt or something first though.