The biggest downside to jk/kj mapping is that words that end with k are somewhat common. So a few times a week I will type something like "splunk" and want to exit insert mode immediately so I mash jk/kj after the k. 50% of the time the j is first so I end up with "splun" (kj is <esc>) (k moves up since we're in normal mode now).

The advantage I found of jk over jj is that, in normal mode, jk is a no-op, so if I hit jk as Escape when I was already in normal mode, it doesn't matter.

The only problem is that when I'm editing text out of Vim, I end up with the occasional jk at the end of my typing. That almost happened while writing this comment.