I use mobile dictation frequently since I prefer typing on a full-sized keyboard over typing on a mobile phone screen. I find it works pretty well for me (using iOS 12 on an iPhone 6s). If I am in an area with poor connectivity or in a noisy environment, or if I am using unfamiliar words, it may not work as well, but for my purposes, I find I spend far less time dictating and occasionally correcting what I meant to say over manually typing in text. I utilize the "blue underline" feature to help train the system to recognize words I say when the dictation software gets confused.