Is kha'zix good

He's not bad exactly, he's just not in a great spot. He's one of the champions that new AD itemization didn't do so well for. He doesn't auto attack much, so critical strike is pretty much useless, he suffered from the Last Whisper changes for armor penetration, and he already did poorly against tanks. Squishy comps he's really good against, but at the same time if you're good at Rengar there's not a lot of reason to pick Kha'zix over Rengar in any of those situations.

I don't know what setup you're using for Kha'zix, but here's what I run if you want to try it.

Flat AD Quints 5 Flat AD Marks 4 Flat Armor penetration Marks 9 armor seals 9 MR Glyphs 12/18/0 Masteries Ferocity: Sorcery, Double Edged Sword, Vampirism, Oppressor (Most people take Natural talent instead, I like the little bit of sustain you get from vampirism as Kha'zix's clear is pretty tough on him.) Cunning: Savagery, Runic Affinity, Merciless, Dangerous game, Precision, Thunderlord's Decree

Item Build: Any jungle smite into warrior. If you're against a champion who is going to want to fight you a lot I take skirmishers for the mini exhaust. Typically I take Chilling smite otherwise, but Tracker's has it's merits as well. I then build into the following items. . This lets you sit on around 35 flat armor penetration after your runes, armor shred from cleaver, 40% CDR, a mix of defensive stats, boots of your choice, and GA is always a great buy on Kha'zix as you're often the focus of a lot of attention as soon as you jump into them. Obviously if you're against all AD you don't want Maw, but if you can find any excuse to build it, do so the flat penetration is huge on Kha'zix.

He's a really good assassin, he's bursty and extremely mobile. R,E,Passive,Q,W,E and you are outta there with a kill. He's weak to hard cc.

He used to be a meta pick, he'd be in every game. Then they changed his Q passive to be a flat amount of damage, instead of %. Since then he has always under-performed; at least when compared to pre-nerf.