I have owned Things for several years now, and it’s too much for me. I’ve attempted to start up with it several times and always get away from it. Now, I use Fantastical 2 and Notes for most everything I do.
Agenda hits the sweet spot for me - it’s the perfect combination of simplicity and functionality. I can do GTD in it by setting up a few categories without all the added complexity of Things - and use it as a great Notes replacement, as well.
However, integration with Fantastical 2 will be essential for me as I’ve noted in another thread. I do not use Calendar at all and without the 3rd party integration, Agenda is only 80% there. Hopefully this can be added as part of the paid upgrade soon.