The MOP is useful when you need to perform lots of class introspection. It's therefore a requirement for a lot of class-building extensions - i.e. most of the MooseX namespace, such as MooseX::Clone, MooseX::Privacy, MooseX::MultiMethods, etc.

That is; you probably want to use Moose if there's some MooseX module that catches your eye, or you need to delve into $class->meta; and use Moo otherwise.