As a developer and designer, you just want something that works and is easy to use.
ACF works really well. It's very easy to add to themes and tempaltes, as well as plugin files.
Documentation is very well written - much like the wordpress codex itself.
In just a few minutes, I had custom fields running on posts and custom posts. The premium add-ons added even more functionality, which was really cool.
Love this plugin. Nice work.