So far I'm very happy with these springs.
I got these and some Koni Strt struts and replaced my oem struts and springs at the same time. I can't say which component is making the biggest difference, but my ride is extremely smooth and cornering is vastly improved. I feel way more in control of the vehicle. So far (1 week ~ 150 miles) I've seen a 3.5cm (1.4in) drop in the front and 3 cm drop in the back. If you want an improvement over OEM performance for cheap and don't want to lower your car 2-2.5 inches these are for you.
H&R springs on Miata
I just installed the H&R springs on my 2012 Miata (sport suspension) the springs lowered the car about 1" at all 4 corners, top of wheel opening to center of hub is now 13.5" all the way around. The ride is very similar to feel of stock sport suspension, however, the car now has very little body roll in high speed corners. I'm very happy with the springs and would do it again.