Palm Treo/Pre and BlackBerry users! And probably Clicks users too. It's not a matter of "Does Russian language use the Latin script?" (it doesn't), but rather "What is the least annoying method to input Cyrillic on a BB-style keyboard, which doesn't have enough buttons for the йцукен layout?". Phonetic layouts such as яверты or яшерты were very popular for such devices back in the day.