They nerfed him a lot, he's still good, but situational. If you're looking for good top lane champs to start trying out I would look into kennen/vlad. Both very very very strong.
I for one suck at jayce and i play top lane. I have never really understood his damage output or how he works. But i have seen him do quite well. I think its sort of a combo you have to use. some people you use a certain combo to get the best trade. and others you use a different combo. i realize that this implies to most champions, but thats all i can give advice about. other than expeirement. do a combo and see who wins the trade. if you lose. use a pot and try another way.
His damage output is poke. You use your gate and shockblast to poke the target down, then switch to melee form and go all in to finish off the kill. If you go all in first, you will get instagibbed or killed or pushed out of the fight every time.
Your experiences are typical for the most part.
The way I have built Jayce is just like an ADC. It's nothing different, only he has the capabilities to buy the Ravenous Hydra and the Runaan's Hurricane, and still be available for the passives. Jayce is a good top laner, just build him with a lot of health, and damage. Then, like you said, he becomes a "Murder Machine".
-Warmog's Armor
-Blood Thirster
-Runaan's Hurricane
-Bezerker's Greaves
-Ravenous Hydra
- *Late Game Item depending on game play*
Just some notes I have gathered from playing Jayce. If you need to know more, ask some more specific questions!
There is no point in ravenous hydra as he is not melee form for the most part. Warmogs isn't really viable either, although situational. He is an ad burst caster meaning greaves isn't really worthhwhile. Once again he is a burst caster so hurricane isn't really used to full potential.
A typical Jayce build involves
-blood thirster
-black cleaver
-last whisper
-guardian angel
-tabi/merc
-Infinity edge if you're extremely fed
-Frozen mallet if you need hp/situational
-Triforce if you're fed
-2nd blood thirster
etc