For me, Sundays are all about taking it slow and enjoying a really good cup of coffee. Mercury fit the bill perfectly; my cappuccino was crafted well and tasted delicious, and I felt comfortable chilling among the clusters of coffee-consuming couples.
The space is much smaller than say, Te Aro, and does have a markedly different vibe -- more indie-retro than rustic and homey. I found the baristas here more on the gruff end of the barista spectrum, but not in a seriously off-putting way. Also good to note is that wifi is free (look on the blackboard for the password to avoid angering the baristas by asking) and street parking in Leslieville is free on Sundays.