I do.

iPhone 6 + Prompt (a SSH client) + tmux + Neovim/Vim + a remote dev server

for Android phones use JuiceSHH instead Prompt

Usually I use a notebook sshing to my dev server but when I am on the go and get good ideas I immediately try them on mobile before getting home. With tmux I am still in the same session having easy access to all windows and panes. Vim's key bindings are the best way to use an editor on a phone anyway. All very satisfying and not much slower. Sometimes when I need to go to bed and still want to play around with the code the same: I tinker a bit more till I fall asleep.

Or today: I read about a new JS NLP library on my phone I want to quickly test: I just start Prompt and type "npm i nlp_compromise", fire up Vim and play around.

I am quite fast with the iPhone keyboard and I think the reason is that I use the keyboard in general without any assistance, so no spellchecking, prediction, etc. Just the plain keyboard and after a while using it like this you get super good and fast (it took some weeks though). The keyboard of the iPhone is also one of its features I like most: it's extremely responsive and never lagging.

Prompt extends the iPhone keyboard well: you get Ctrl, Alt, Tab and more keys nearby and aesthetically very similar which I like. Prompt is quite slick and iPhone's iTerm.

So, it's not about doing entire projects from A to Z on your phone but it can get ultra productive when your are away from your computer and get those 'shower thoughts' you haven't gotten for three hours sitting in front of the screen. Then a phone as a devkit comes quite handy.