Before my summer vacation I purchased a power bank. I did this because year before I discovered problems with my nexus 4, which I was a key device in the travel - navigation, hotel reservations etc. Sometimes, but not always, it was charging slower than using battery during driving. And it was rather unpleasant experience.
But this year my main smart phone was new moto g. And know what I used my power bank once. And even that time I did this to be on the safe side.
Moreover, using nexus 4 I usually plugged it in my laptop for charging when came in the office.
Moto g changed my habit - I do not charge my phone in office any more.
What about power bank? It still with me in my bag, but I’ve never used it.