Is he still viable? He's sitting on something like a 47% win rate according to lolking, but I'm not really sure how reliable that is. I know his E was nerfed VERY heavily along with the mana costs on his shockblast combo meaning that he can go oom very quickly if not careful. Manamune seems to be popular at the moment, but does all of this really equate to such a sudden drop in usage and success? I'm probably the furthest thing from an expert when it comes to things like this and would definitely appreciate some insight from the more experienced players, but it seems like the nerfs simply stopped him from stomping his lane opponent everytime they tried to trade with him, I mean, he can still prevent or at least delay an enemy from following up on an engagement with his knockback. Your thoughts?

He still a very solid pick atm. He was picked and banned atleast once during the European lcs qualifiers and the Brazilian Keyd team was running Jayce in their infamous win against the Korean LG.IM.

They played him in the mid lane but since BC was nerfd once again I'm not sure if the results would've been the same but he's still a strong mid champion (I recommend trying him mid against traditional mid champions with little to no escapes like Twisted Fate).
I don't think he's a good duelist because he lacks any consistent burst damage or cc most popular top champions but his poke is still really good and shouldn't be underestimated just because he got nerfd.

Lolking and winrates often have little to do with whether a champion is good or not. Take Olaf for an example. His winrate was under 50%, said Lolking. But then he got used in tourney level play and destroyed things there. Then he got nerfed. Why could this be?

Obviously, Lolking doesn't factor in player skill, the champion's allies or the enemy team's performance. Nor does it take into account trolling, afking, or whether you were building the correct items for the game in question. So why use it for anything? Sure, if a champ has a very low winrate, there may be something wrong with them, but Lolking cannot tell you what that is.

As for Jayce, he's fine. He's got two different build paths (AD caster or bruiser), has excellent poke damage, good burst at all stages of the game, and also has one of the stronger laning phases in a solo lane due to his ranged harrass and ability to follow that up in melee, much like Nidalee can.

He might be relatively tricky to get the most out of, but having a winrate under 50% doesn't actually make him bad. If you're going to use winrates as a judgement of power, you also need to identify weaknesses in the champion to see how bad they are. If you look at Fiora (who has a terrible winrate), you can clearly identify a large number of serious problems she has (forced to build damage, but melee with no real durability. Weak laning phase, weak passive, and a need to snowball to be a threat). When you compare those to what Jayce has (which are mana - solved with a manamune, and being relatively weak when spells are on cooldown - like any AD caster), you see the difference.