I needed a different kind of chair for my lower back issues, and decided to take a chance on a kneeling chair.
I would say while this wasn't the easiest to put together, it wasn't terrible. Just don't go in expecting a quick assembly and only needing to insert a bolt or two.
As far as the chair goes, it took me a few days to start to get used to it - my knees/legs would ache a little if I were on it for more than a few hours at a time, but after I got used to it, it's fine for a full day of work.
It really does help with your back. I was surprising how much better and more supportive I feel while in this chair, and feels so much better than even a decent standard office/gaming chair. It does a great job of having you maintain your posture.
If you have issues with your lower back, I'd say take a chance on one of these. I now love mine to death, glad I took the chance.