Hi, my name is Rob, and this is my first guide for Mobafire. I'm here to share my knowledge about Jayce that I've learned from over 200 games played with him. Play Video
I've tried several different variations of builds for Jayce, but have found the most success with this one. Originially, I tried Jayce as a tank, building several tanky items. Now I build him more offensive, aiming for more damage in team fights and hoping to stay in the back line. This build shows how I build and play Jayce.

Pros
+Strong burst
+Good at laning
+Great poke late game
+Good at pushing down towers
+Has 6 abilities that he can use (not including ultimate passives for extra auto attack effect)
+Can give his team a disengage with his Acceleration Gate

Cons
-Has no form of knockup or stun CC, only has knockback and slow
-No Innate sustain making it harder to lane against those that do, especially Irelia
-Not that tanky when in Cannon form
-Not a ninja

I run 19/11/0 for Jayce in order to completely shut down the enemy in lane. Late game, masteries aren't going to matter as much except for the Magic Penetration and Armor Penetration. In lane, you need to take the minion damage reduction and health in the defense tree in order to poke the enemy with auto attacks and not take as much damage yourself. In the offense tree, you don't need the critical strike damage increase because you won't be critting that often with only Trinity Force. I take the magic penetration through cooldown reduction due to the high amount of magic damage that his E does in Hammer Form.

For my runes I use 15 AD in quints and reds in order to poke the enemy champion down in top lane. These runes work extremely well with Jayce's Q and E for good burst and poke in lane.
I use magic resist blues when I am playing against an ability power champion in the top lane, and also this is effective for taking any harass from champions that have both physical and magic damage. Against people with no magic damage in the top lane, I run scaling magic resist blues in order to tank more damage from the enemy AP carry during team fights late game.
For yellows, I run armor for obvious reasons. It gives good armor for playing against physical damage dealers, and it helps against auto attacks from enemy champions even if they are AP.

Jayce is strong against:
-Darius
-Jax
-Singed
-Shen
-Teemo

Jayce is weak against:
-Irelia
-Yorick
-Xin Zhao (be very careful if he hits level two before you do)