`kj` has worked really well for me (except, actually, when trying to write a blog post about a recent trip to Reykjavik, but that's the one time it's bitten me versus the shitton of time it's saved). I also have `zkj` mapped to `<ESC>:w<CR>` to save AND exit insert mode.
It's the combination jk (or kj) that is set up to switch from insert to normal mode. The idea is that this combination rarely occurs in English text. The drawbacks are a slight delay in cursor movement when typing j or k as vim waits to see what comes next, and momentary confusion when you inevitably need to actually insert these sequences.
I much prefer mapping Caps Lock to Escape at a different level (with xcape -- caps is both my i3 modifier and my Escape key). That way, when I SSH into something and use vim or a shell it still works (or when I vim as root), and anything with a vim mode will just work.
I had jk mapped for a long time and thought it was safe. Then one day I actually had to type it as text, and it took me a moment to figure out what was wrong. I'm retraining myself to use ^[ now.
I don't think the speed hit you take when you actually have to type 'jk' should be a dealbreaker. It rarely comes up, and when it does, you can use e.g. '<C-v>j<C-v>k' or 'jjkak'.