Ask HN: What a/b testing framework do you use?

I am currently looking into using an ab testing framework for our company. We want something that is framework agnostic, preferably with a REST API but also with good language integrations already. It would be great to have tools to target tests at specific user demographics based on our user data.

We don't really want to build something from scratch.

I have been checking out Optimizely so far and it seems like it will tick most of the boxes. What other alternatives should I be looking into? What do you love and or hate about what you are currently using?

Interested in both paid subscription services and OSS