You'd be surprised at what great finds there are in the Manulife Centre! I frequently find myself stepping into this mall throughout the week for trips to Jacob, Indigo, Cineplex, LCBO, Bloor Street Market, Pharma Plus... and the list goes on!
It also connects to the subway, the Holt Renfrew Centre (which is also full of great shops), and Cumberland.
I would say its sort of a smaller version of the PATH, that connects you to Yorkville.