I am one of the first users of Freemius, and it's a lifesaver for me. After that, i recommended Freemius to so many themes and plugins authors, and all of them are actively using it to date. With Freemius, I was just able to focus on product development, everything else, like subscriptions, affiliates, payment, licensing was handled by Freemius. Vova created an amazing service, thanks a lot.

I also wrote a blog post back then, https://www.wpentire.com/increase-wordpress-business-freemius/

Also, I was able to sit and talk with Brandon about Freemius and he shared something great about Freemius too, https://www.youtube.com/watch?v=ox98hK7XKU8

Go and use this, you will never look for alternatives.