I just installed a Spec stg 3 w/aluminum flywheel 2 weeks ago. I hated it, and I mean HATED it at first. The clutch was very touchy and grabby. It also chattered a lot when trying to start off in first.
Now... Two weeks later... I'm pretty much fine with it. It has more slippage now and chatters much less. You really have to break in the Specs properly to get rid of the chatter. I've got about 350 miles on mine since the swap and its getting better every time I drive it. I drive it daily on the street and it took some use to because its so grabby, but now its very managable. And boy does it hit HARD. You have to be carefull because the Specs, especially the stg 3, will really shock the drive train hard.