I had a 9lb Fidanza flywheel in an old car (non Subaru.) It totally changed just about all driving aspects of the car, but mostly for the good. The only "bad" was the learning curve once installed, but it was really no big deal. Driving in traffic was hardly affected.
The biggest change was needing to rev match every shift, because the revs dropped as fast as they climbed. Just a blip of the throttle could take you from say, 3k to 6k, so learning to properly rev match was needed as well. With the A/C compressor engaged it was even more exaggerated.
It had a replaceable friction surface, so I don't if that counts as composite, but I drove tens of thousands of miles without issue. Idle was dead on, and there was no clutch chatter.
The pros FAR outweigh the cons in my opinion, however I can't speak for experience on a Subaru engine.