Currently running on factory default Bilsteins with Mazdaspeed lowering springs and Rx8 read/yellow sways. It is definitely an upgrade and to my biggest surprise the road comfort is not compromised at all just way less body roll. I think it is perfect if you do not want to track your car. Yet I'll move to Tein because the Mazdaspeed springs simply too soft (about 2.5/2 kg/mm) for trackdays and autoX and it is inconvenient during the race to feel that the spring rate change three times (soft spring -> hard sways -> bumpstop).
As I also use my car to commute, had to try Flex first. Fortunately, a local club member offered his NA for a short test drive. His car is equipped with Flex (for NA the springs a little bit stiffer: 7/6 kg/mm f/r, Flex for NC comes with springs 6/5 kg/mm f/r) and it was fairly good. It was interesting as I suspected that my spine will have some severe damage but no. The ride was definitely very firm, tire noise (when tyre hit something on the road -> pothole, asphalt joins, dilatation joins, etc.) increased and I could feel that the car hit the ground much harder but somehow it did not reach my spine. However, it is not the suspension I'd choose when the roads are bad in your area. In bad roads (also tried here: https://email@example.com,19.188673,3a,75y,105.33h,55.75t/data=!3m4!1e1!3m2!1s3Qus4EawhJGNvtq-Bv5xfA!2e0
) it is simply not enough comfortable for commuting.
I also tried EDFC and it seems it makes no sense on the public roads as the springs are very stiff for normal (or even spirited) road driving and the difference between the hardest and softest settings could be felt only during hard cornering.
@Brian: As far as I see Flex for NC has very short rear shock travel. How much preload you suggest to set? As far as I know normally 0 preload is suggested but it also applied to the Teins too? (And of course thanks for the info you share with us, very-very useful!)