Last summer I fitted some Eibach Pro lowering springs along with wheel spacers to my ND. The wheel spacers are 15mm front and 20mm rear. The main reason for fitting the springs and spacers was for looks as now the wheels fill the arches much better. As a bonus I have noticed the car dips, dives and leans less in bends, overall the car feels more sure footed. From a comfort point of view I can honestly say I have not noticed any real difference, the car still rides nicely and handles bumps and pot holes very well.
For anyone considering making similar changes to their car I have added before and after pictures so you can get an idea of what results to expect. The parts came from MX5parts and fitting was fairly easy taking about a morning to complete. Other than a good socket set, jack and axle stands the only specialised tool required was a pair of spring compressors which I purchased from Halfords for £29. After allowing the springs to settle for a week I had a professional four wheel alignment carried out. Then when the car went in for its first service a few months later I asked the dealer to check the headlight alignment at the same time.