Hmmm hasn't been whole lot of discussion here! I've had some good info from over at NSCC forums though. Is public forum I believe, but you will still have to register to see:
http://www.nissansportscarclub.org/bbs/vie...2060&highlight=
The basic info is that lightened crank pulleys can be an issue on cars with more than 4 cylinders, and also engines that are not internally balanced - the stock crank pulleys have a dampening/balancing effect on the engine which can be lost by the lighter weight and lack of dampener on most lightened pulleys.
Underdriving the water pump, etc. separately can be a wiser move.
Alloy crank pulleys can also cause premature crank seal wear which stock cast iron ones don't suffer as the seal can bed in better or some such thing.