Erin Mills Town Centre is unfortunately just a "I need to go get something and it's the closest location" sort of situation. I would never choose to go to Erin Mills Town Centre on a day off (unless I'm feeling particularly lazy) especially since Square One Shopping Centre is just an extra 10-15 minute drive. Most of the stores here you don't know when they will be leaving the mall as their is not enough traffic to keep them in business.
There's really not much to see and there is a lack of variety.
They do however, sometimes host events in the middle of the mall which can be fun, but it's rare and far between.