I'm still Google-OS free after nearly a year of Firefox OS.
The inbuilt email client mightn't be quite as slick as the Gmail app but it's way more usable than Google's mobile mail web page. Facebook feels lighter than the app on my old HTC. I use the web interface of the old reader since I never found an RSS reader on Android I was comfortable with. Here Maps from Nokia does the job, even if it's not Google.
A heavy app user would feel cheated, I guess. But I'm a cheapskate who never *purchased* a single app from Google Play and 90% of the time I am more than content with a dumb phone running on bleeding-edge Gecko, which is smoother than Android browsers on similar hardware.
That HTC runs Kitkat via cyanogenmod, which I've since donated to my 73yo mother. She finds Android heavy and confusing - her daughter-in-law thinks she'd be better off with a shiny new iPhone. As an experiment, we recently swapped handsets - she found Firefox OS cleaner and more intuitive and was reluctant to give it back after a week, except for the apps she'd be missing - mainly from MS (Office for viewing mail attachments and Skype)