For anyone interested in knowing my experience, I own both the DJI 3ND FIlter set and the PolarPro Cinema Series 6-Filter Set. Let's talk about the weight, the DJI ND set are almost nonexistent so very light, but the Cinema PolarPro's are also Extremely light and not much difference from the DJI. I have seen gimbal errors with other 3rd party filters but never with my cinema series. As for construction quality of the DJI ND set, I'm sorry to those who disagree, but they're flimsy. A lens popped out of my ND8 just gently un-clipping from Gimbal (I was able to pop it back in but what the heck?). I've also heard of multiple people breaking the delicate plastic tabs that hold them in place. On the other hand, the Polar Pro cinema series are just way better made with HQ materials/Glass (aircraft grade aluminum or whatever) and seem like they'd be tough to damage and come with a nice storage case (Thanks DJI for including a piece of foam to store your filters in).
As for the quality of video, The DJI ND filters do what they're supposed to and are adequate for adjusting the Fstops associated with 4 (almost useless IMHO), 8 and 16. But (again IMHO), the Polar Pro cinema series look light years better both before and most importantly after post for color correction. I've flown repeated way points in various conditions comparing the DJI ND's with the corresponding Polarpro ND and CP filters and Polar Pro get mostly subjective (but some objectively) better results in all situations. And how about the option of using polarizers for certain filming preference which can make the colors "POP" and look way more vivid than the Neutral filters or reduce glare off water/snow (Plenty of Youtubes out there show this, just Google it). OH, and how about having a ND32 for super bright sun or mid-day filming. Also, The Polar Pro's Cinema filters sleeve completely surrounds the Mavic lens vs. the DJI ND set does not and may let light through the sides between the tabs to permeate between the Mavic Lens and filter. Anyway, I'm no expert but have used all of them and have a clear preference for the quality and variety of the PolarPros. I believe it's worth the extra $$$ and would have not bought the DJI if I could go back. I only bought the DJI set because all the initial 3rd party filters were having trouble with gimbal initialization and overload errors, etc in Dec/Jan, but PolarPro at least now asserts all filters are redesigned to avoid this and work fully with the Mavic gimbal.
I'm sure others obviously feel differently and DJI Nd set OK for them, but I never use mine anymore and would happily sell them at cost.
JC