MEC is heaven for outdoors enthusiasts. They sell tons of gear here for all kinds of activities like skiing, kayaking, camping, biking, etc and offer maintenance for equipment like bikes and skis. The selection and quality of the products is fantastic. I have bought lots of clothing, backpacks and so on here over the years and have always been impressed by how well everything holds up despite lots of use.
Another feature worth mentioning is that you can rent gear here like snowshoes so if you're tempted to try a new outdoor activity but don't want to buy anything until you have tried it and know you want to commit to it consider renting. There are also a variety of other services like gear swaps and recycling of goods like bike tires and climbing ropes.
Its one downfall is the location. It is not very accessible by public transit and Marche Centrale can be a zoo at times (especially on weekends). Navigating the parking lot and finding parking during these peak times can be annoying. If you can try and come during the week when it is quieter.
You have to be a member to buy anything here since it is a coop. Membership is $5 (at least it was a few years ago when I joined) and is good for life so its not an expensive commitment at all. That tiny fee is well worth it though because you will definitely come back time and time again.