Well talking SMF, i've just fitted one to the avant, which is predominantly driven by her indoors. Its a standard Audi flywheel, not lightened, just redrilled to take a bigger clutch and balanced up afterwards.
I've had one on mine for a year and its mostly ok, but she drives differently to me, using lower revs and a much more relaxed style. I find that now the car has a SMF, it makes an awful reverberation/drone that vibrates thru the car when under load below 2krpm. Around town, 30mph in 4th gear is just awful, The motors only doing about 1600rpm, and even the slightest amount of load applied and you get the drone, find a slight incline and the noise is so awful you basically need to change into 3rd. Similar thing happens at say 40mph in 5th if you find a bit of an incline.
My car does the same, but i tend to drive in lower gears with higher RPM, so i dont notice it as much. Plus i've conditioned myself that as soon as i apply load and hear the noise i'll change gear without thinking, whereas she doesnt as shes just wafting along, not trying to boot it up a hill.
Thats before we even get into the issues you get when you lighten them. Chatter, less inertia making it easier to stall and requiring more clutch effort when drawing away or driving in very slow moving traffic, more RPM drop on gear shifts etc etc.
As i say, its an easy way to ruin a nice car. On a loud rattly harsh focussed track or weekend toy, it wont matter a jot, but on a "full fat" everyday car, it certainly needs some thaught before just jumping in and changing it.
