Pros: they free up parasitic loss from the stock pullys. Cons: they slow down the alternator so it's turning less and not making the same amout of voltage. From what I read people pushing HIDs, and stereo systems (1000watt) doesn't have any problems.
UNDERDRIVE PULLEY KIT FOR 2002-2008 Ford F150 5.4 and 4.6 with 6 Rib System http://www.troyerperformance.com/cgi-bin/ppistore.pl?user_action=detail&catalogno=6RUD5446