I'm a pretty paranoid parrot when it comes to diketones in my e-liquid, and it seems people bring up the topic everyday. There's a lot of misinformation being perpetuated, and to me it's like, "if we don't know, i don't wanna be the first to find out." Dr. F even calls it an "avoidable risk".
I try and stick to fruity juices (which supposedly may also have tiny bits of diketones in them, granted, far less than a custard butter popcorn juice) and NET's (which also may have diketones in them). I have considered going to unflavored, but really a large part of the appeal of vaping was the flavors. I fear I might go back to cigarettes if I don't have that nice fruity menthol taste in the morning.
Either way, I do have some juices in my roster that I enjoy, and I'm positive have some form of diketone in them. I just try and limit the amount that I actually vape of these juices. Like, it's a once every couple of days sorta treat. Something to have with a glass of single malt (Lagavulin, preferably). One of which is Nicoticket's Cheesecake, which I bought on a whim and steeped for a good 3 weeks. It's really tasty now though, but I can definitely recognize that creamy hint that just has to be bad for you. It's almost "too good to be true"? That said, I've tried some other custard juices (in particular, Epicus Nebula's "The General") which tasted downright poisonous. There was an undeniable chemical note and I can only assume that it had to be something bad for me. I told friends not to vape it, and this was long before I knew about diketones or diacetyl or AP. It just didn't taste right. I've mentioned it, and the maker assures that "there is no diacetyl in my juice".
So like when someone hands me a bottle of juice and says, "It's a custard but it's diacetyl-free" I recognize it as a perpetuation of misinformation (nice rhyme) -- just because it doesn't have diacetyl actively used (the maker didn't put in 3 drops of diacetyl when he was making the batch) it probably still has diketones naturally occurring in it somewhere.. People tell me, "I've been vaping custard juices for a year, and I haven't noticed any negative side effects!" Yeah, well not EVERY worker in the popcorn factories got popcorn lung, and it took a substantial amount of time to manifest in that, the most EXTREME of situations. Who knows what we might found out 10, 20, 30 years down the line? What if trace amounts (PPM or even PPB) directly inhaled into the lungs can be negative over time?
Anyway, enough rambling. I dunno, it just seems like an "avoidable risk"/I didn't quit cigarettes to pick up something that's gonna make my lungs go ker-splat, y'dig?
ETA: I've spent a lot of time arguing it with people, whether it's actually a risk to be concerned about or not. The truth is, "nobody knows yet." So why take the gamble? You could go your whole life smoking cigarettes and manage to not get cancer (ever seen those perfectly-healthy 90 year old 3+ PAD smokers and wonder how they do it?) They took a gamble, and they won. You could do the same, but you switched to vaping to find something less risky. It seems self-defeating to then pick up something which may (or even may not) have some future health implications.
You know there's gotta be people who, before everyone knew cigarettes were carcinogenic, said to themselves, "This can't possibly be good for me." I'd like to think that I woulda been one of those people.
*takes a puff of cheesecake*
Whether that be certain movies or certain eliquids. 
