I've gone here many times. I love how it opens until 12-ish so when I'm done having dinner I can take some time to check out books. They have a good selection for a used book store (this is my favorite of the three BMVs in Toronto). I've found decent things in the $1 carts, although digging through it is kinda musty. What I don't like is that there's no catalogue system, so when I came by to check for a copy of Godel Escher Bach I had to look in three sections (religion, science, philosophy--and still didn't find it).
Another thing is they buy back your old books, but only if the books are published recently and they only pay about $1 each. It's useful, but not really worth it if you're coming all the way downtown to offload 10 hardcovers if you're only getting the bus fare back.