I know your going to down-vote me for saying this but... Jayce needs a buff and VU.
Let me explain why. Jayce draws a great deal from switching between Hammer and Cannon to get that extra burst of damage. The problem is he cant use his hammer anymore. Melee for Jayce is a death sentence, you may dish out a whole lot of damage but even low HP enemies can just blast you. Half of his kit in a nul point right now because he simply can't survive in melee combat due to the inescapable fact that he has very little defensive ability even with his hammer buffs. I have been testing him for a while now and he has too may problems.
While his AD ratios are high he just doesn't have room for full AD in his build. By design he has mana problems its one of his key balancing points. The problem is that his ideal fix for this Manamune gives very little AD even when maxed out. He has no clear build path anymore as hes cough between Marksmen and Juggernaut with out the bounces of either.
Lastly he has very few indicators on his skills. His Hammer Blow that sends enemies flying in a direction away from him has no indicator as to exactly were they are going or how far. I would not even bring it up if not for the fact that Alistar and Vayne have very similar abilities and have those indicators.
I just think Jayce should be given a good look over by Riot and given what he needs to see good reliable play. Now I know I will get some Jayce mains on here saying hes perfectly palyabule right now and there is nothing wrong. I can say I agree, he is playabule, he damn fun actually, but he just doesn't have what he should right now.
Also pleas excuse any grammar or spelling mistakes, I use a program called dragon to type long posts and its hardly good at translating my voice into words.