1.) The same thing as Rengar, but different - Instead of playing a Predator, I'm playing an Alien. More than that, he's one of the more satisfying Assassin's to play, because his kit is so deceptively simple (Compared to someone like Fizz or Zed), but at the same time has a lot of outplay potential and little kinks to play with, like using your ult right as someone walks into and out of a bush to juke them, or ulting through someone during a duel, etc.
2.) I actually had a really, really good Kha'Zix game the other day, and what frustrated me was, late game, my Q wasn't doing the job it should have. It took half of the Kalista's HP, and we didn't have a tank. I was fed that game, and I was pretty much always zoned to the sidelines. Perhaps this is true, and should be true, of all Assassins, but without another target for them to focus on, I was much less effective. Typical things like isolation radius and such didn't bother me. Another thing that did, though, was his passive, which I actually found really difficult to proc when trying to Assassinate, but again, may have just been because I was facing a Kalista.
3.) I don't feel like he's overdoing anything. If anything, though, it would probably be unhelpful utility. Evolved Void Spikes reveals a foe? Great? It's never been very helpful. It's so unhelpful, in fact, I feel as if I'd rather take evolved R than evolved W. I feel like, with unhelpful utility like that, that Riot may add more unhelpful utility to cement him as the 'utility Assassin' (Which I think he should be). I think he's missing selfish utility. He has it in some places - Increased range on evolved Q and resets on Leap, for example. But there isn't enough in his other evolutions. Evolving his skills is hardly a choice beyond the first one (Which is a toss up between Q and E). He needs more selfish utility to cement him as the utility Assassin who can evolve to fit any circumstance.
4.) I love Armor Penetration Assassins. Much, much more than AP Assassins or crit based ones like Critgar. I'm not sure why. So, I like building ArP items on Kha'Zix. I actually like Duskblade on him a lot, and I can pinpoint moments when I don't have it where I wish I did - Maybe I built Maw selfishly, when the extra damage from Duskblade would have secured a kill for me.
5.) Thematically, yes. The way his evolutions are implemented is fantastic. But functionally, no, because their's no choice in his evolutions. It's always either Q or E first, whichever one you didn't choose, second, and then W. No one considers R anymore. Certain evolutions are mandatory and the others are more obligatory because they help a little, but not enough for them to be noticeable. It makes him less of "Evolve to meet any situation", and more of, "Reach your evolutions and late game as soon as possible for bonuses."
. He'd be funner with the option to place more of a emphasis on his fighter secondary (Lower burst, better dueling / survival). At the same time I'd like kha'zix to still be able to go pure assassin style with something like 
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