This is NOT just a spin-off/filler series. There is a real story here AND it adds to the story told in season 1 of DanMachi.
It introduces a few new characters and fleshes out some that only had cameos in S1. Plenty of good story and character development.
I had low expectations going in, BUT, my opinion quickly changed as I got about 3 eps in.
I'm actually hoping for another season of this.