I have a 8.8 lb ACT prolite in my GSR. The throttle response is much improved and rev-matching is a whole lot easier because it revs up and down very quickly. I haven't noticed any drivability issues because of the flywheel, only that it is harder to take off on hills and it takes a little more gas to get going.

I have heard of them breaking, but only with 400+ hp cars while road racing, so I don't think that is anything to worry about.