The standard ND filter that comes with the Inspire1 is helpful, but not good enough on bright days. Basically the rule of thumb is to have the shutter speed double what the frame rate you have set. I usually film in 24fps the shutter speed should be 1/50, the reason for this is it gives the most natural motion which is more pleasing to the eye, and also if your shutter is too fast, then you can suffer from the Jello effect, so getting the shutter speed right is very important.
I have purchased a set of ND filters in the range of ND8 to ND64 by a guy called Renaat from China. The beauty of these filters as opposed to others out there is that the weight is identical to the original ND filter from DJI, and this is important as Gimbals are delicately balanced as slight shifts in weight can have adverse affects. I remember this being the case when I put a filter on the 12mm M-zuiko when I had a GH4 attached to the Zenmuse Z15, smallest of weight differences just threw out the balance of the gimbal and caused drifting and yaw issues.
After using these filters I am massively impressed. I like to use a custom white balance on every flight, and I am seeing no colour shift issues at all, no flare issues or sharpness loss either. Also a nice touch are the dots on the side of the filter to tell you which is which.
Overall a fantastic set of filters and I see no reason to go for anything else.