To put it into perspective: He's a new champion. People are still trying to figure out how to play him. Lee Sin was considered terrible until they got the hang of him, then he was overpowered and nerfed.
I think I've got him down for the most part, as a Swain who had the horror of laning against me top may attest. Jumping from bushes with an Empowered Q, which hit him for approximately 250-300 damage at level 1 of the skill, smacking him with another Q to nearly kill him, then Bola and W to finish him off. All in the course of about 3 seconds. If the jungle tries to gank without having some form of vision, saving your ultimate for escapes is immensely preferable to dying for no reason. I don't think I ever got wards when playing as him, though the Swain bought some invisibility revealing ones for ganks. Other than that, the ultimate does feel fairly lackluster, but it's great as an early warning system for teamfights, and a "I'm going to target you and there's nothing your team can do about it" ability for sneaking through the enemy team so you can single out the squishy while everyone's attention is focused elsewhere is nice.
His burst is godly, but his sustainable damage suffers from "How much CDR can I get without sacrificing survivability." I found that grabbing Merc Treads, Bonetooth Necklace, Frozen Mallet, Ghostblade, Atma's and Zeke's gives him a good balance of survivability and damage output, as well as some much needed CDR. With my current CDR blue's, I get the full 40% reduction with this build and 3 kills on the Necklace. With the current Reds, Quints and Masteries, base 31 Armor Penetration, +10 from his Necklace, and +20 from Ghostblade. It does feel a lot like I'm building Olaf minus one item, but he feels much less squishy when I do so. The fact that his Necklace also gives a base 10 AD + 2 per level means that for 800 gold you've got an item that scales almost to a BF sword at 18. It does feel a bit lackluster as well overall, though, at least until you start getting kills on it.
Trinity Force builds capitalize on his burst a lot more, but he falls apart when focused, which is not something you want to have happen as a melee bruiser. I may work on trying to work this into a build, but it may come at the cost of Frozen Mallet or possibly the Necklace... or just survivability in general. Warmogs/Mallet and Atma's should be core on his build, really, since he needs the health.
The buffs will definitely be welcomed, though. I do have minor issues depending on who I'm laning against top, in particular Teemo because fuck that fuzzy little boyscout. Other top laners will usually be melee and easier to position against, as well as abuse the 15% health heal on his Empowered W.
I have not been impressed by Rengar. Probably because I recently pwned one as the Taric half of a Taric/Ashe laning duo.
His main problem was that he isn't really meant to be laning
with someone. He's great as a solo and as a jungle, but falls short in a duo lane due to being bursty and nothing else at low levels. If you have a way to counter that burst or stop him from performing it, then he's effectively useless. Taric having a heal and a reliable stun is pretty much a double counter for him, not counting the perma slow Ashe.