I paid for it, then got a notification they were giving me a year for free (or maybe lifetime, I don't recall). When I changed phones I didn't install it, then did and now think I'm back to my free/introductory year.
I did use it a lot but honestly not that much now so don't care, if I have to pay again in a year I will...if anyone I know starts using it again. Mostly everyone just uses their carrier SMS/MMS now, I gave up trying to have them use GV and WhatsApp a long time ago, a losing battle it was.
But yeah, it does expire and you have to buy it yearly. It's cheap and a good service.