The Mitsui Outlet Park is located just before the KLIA2 terminal. In concept it is a Premium Outlet selling branded items similar to the Johore Premium Outlet. It is basically a F&B and Retail Outltes for branded items which can also be found in Pavillion KL.
The place is too far from KL center. Even travelling from Seri Petaling (Southern corner of KL) to KLIA2 incurred toll charges of RM16. Travel time will be 60 minutes by MEX Expressway to and from MOP. Besides that the parking is also very expensive. The parking charges is RM2 per hour even on week ends. Hence it will cost you RM8.00 if you park for 4 hours. The 4 hours will just be enough for window shopping and having your lunch / dinner there.
The F&B outlets includes Hidang Restaurant, Coffee Bean, Straits House, The Loaf and other restaurant still renovating. Basically the Food here is expensive and limited choices.
In conclusion, its better to shop in KL provided you're a tourist. Suitable for Tourist doing last minute shopping and provided your tour bus is park outside waiting for you. But even tourist will be spoilt for choices at KLIA2 . KLIA2 has a shopping mall inside and you can just walk to your departure hall after shopping.