I've had one fitted for well over a year now.
My car is a 997.1 C4S so I had to also fit a 3G accelerometer which just involves splicing a couple of wires into the main ECU wiring loom. Turbo models already have this fitted as standard as do the gen 2 models.
The difference is quite noticeable on any of the 3 settings.
The range of the suspension travel increases, without compromising cornering or braking in any way.
Setting 1 I rarely use as I find it a bit too soft and wallowing but is useful on very poor roads or farm tracks !
Setting 2 is the norm for me, very much like the normal standard setting but much more compliant ( like having a range of 3 to 8 instead of a 5 to 7 range ) if that makes sense. If you brake really hard from high speed the nose diving attitude of the car is all but eliminated. Less roll on corners and more comfortable on rough roads.......it really hugs the road better.
Setting 3 is the stiffest setting equal to the sport setting of standard, but totally usable on public roads unlike the standard car setup which is totally unusable on public roads.
Once again the range is extended like from 5 to 10 instead of 8 to 10.
This setting I've used on the track and it just seems to get you in and out of corners faster and under more control even if your not a great driver, like me !
My car setup is standard apart from non N rated Michelin Super Sports.
The only downside really is the cost of the unit, but I would buy it again.
At some point I may buy the DSC suspension kit to go with it, again that's a little pricey at about $8000, hard to justify for my use.
All in all a very good and subtle mod and would probably work very well with lowered springs taking some of the bumpiness out of the ride and keeping the wheels firmly in contact with the road.
HTH
Dave