That depends ... what sort of things might you be interested in / how much luggage space and weight you have / whether to justify a whole day (wasted), and the expense to get there etc.
There are many places in the city to buy things cheaply. Maybe have a look at these before committing yourself : T.J. Maxx, Burlington, Century 21, Macy's etc. I haven't been to those 'Premium Outlets', but a few others across the US and I don't bother any more - especially in NYC where there's sooo much more worthy things to do.