Heres my Jayce build that I've been using since he came out a few days ago and have had great success with it. If you like this build please rate it well and I hope you have good luck with it!
THIS BUILD IS MORE PRIORITIZING THE MERCURY CANNON OVER THE MERCURY HAMMER!!

Pretty basic runes, Armor Pen reds for, well, armor pen. Armor yellows for more armor and survivability. Magic Pen blues for magic penetration on your ability power skills and flat ad quints for more damage.

Focus on your Q first, then rank up your E and last but not least your W. Take points into your ultimate at levels 6, 11 and 16.

PROS
Decently tanky while doing serious damage
Great Versatility (Your team can stick you in pretty much any lane)
Can be an effective ranged carry or solo top with this build

CONS
You may get focused in team fights when the enemies realize that you are a threat
You may cause enemies to ragequit
You'll have to deal with people crying OP as you crush them with your hammer