Mojo is right. Any time you can take weight off of the drive train, especially reciprocating weight, it is an advantage. First, it lightens your bike. Second, sprockets are easier to get. Third, a 520 chain is often cheaper than larger sized chains. There are other added benefits too. Lighter drivetrain translates into less parasitic drag. Less parasitic drag can offer better throttle response. It also takes less of the engine hp to turn the wheel, making it seem like you have added power when you have not. It can even transfer into minimal gains in fuel economy.
The only cons I can see would be increased rate of chain wear, slightly less tire wear, and if coupled with a -1/+2 sprocket combo, a handful of tickets or a night in the pokey....

:toocool: