Ranley K.
Yelp
With the new airport MRT (Linkou Station) just finished, the Mitsui Outlet Park is 1000% more accessible to the masses via mass transportation. From Taipei, take the express train to Chang An Hospital and then switch to a local train there. The times for the local and express trains are usually matched up, so you aren't waiting around for a lifetime in the station for the transfer.
The outlet itself is about a 5 min walk from the station, but if you're lucky, you can hop on the free shuttle bus (which also makes its way around Linkou) if the weather isn't too nice.
If you've ever been to a mall, the outlets is pretty much a mall, but with outlet deals. I noticed the prices weren't any cheaper than the normal retail stores, but there's deals where you buy one get one half off or some percentage off.
A few of the stores will have you queue to get in. I saw Nike, Adidas and Michael Kors all have lines.
The food court is on the second floor and boasts a pretty large amount of seats...for family members to lounge around for the entire day while the rest of the relatives shop away.