Never driven drunk, no plans to. It really is insane.
I have once (accidentally) driven while extremely high. I ate an entire weed chocolate bar and forgot about it while at a girl's house with a friend. After 2.5 (sober) hours at her house I went to drive us home, assuming that it would not take effect if it hadn't already.
About 5 minutes into my 22-mile drive down the highways of LA, I suddenly became extremely high. Like, so high that I could not remember what had happened ~30 seconds ago. I was like a goldfish; I kept looking up and thinking "How did I just end up on the freeway? Didn't I just leave her house?". This was not the common sort of relaxed high driving, this was me pretty much blacking out high (I even felt a bit like puking). I would have stopped, but my friend convinced me we needed to get back home ASAP. Then, at 1AM, we hit bumper-to-bumper traffic. Don't ask me how that's possible, it's one of the wonders of LA. My memory is pretty much blank after that, but my friend kept telling me when I needed to turn and somehow I ended up in a bed the next morning. I will never, ever do that again.