What can I say about Bloor-Yonge station that hasn't already been said. This is probably the busiest station for the TTC other than Union. The fact that it is a connection station makes it even more crazy during rush hour and peak times. You'll need to pass by it though, at least once if you're looking to go downtown.
The crowds here are wild, and people will not hesitate to push you over to get to their train. Subways are often packed to the brim during peak times, and you'll feel like a sardine in a can if you get stuck in the rush hour blitz.
The small shops around the subway (exiting the station) are great though, you can find an LCBO, Shoppers Drug Mart, and even a Dollarama nearby if you wanted to grab some things before you head on your way. Overall a busy and fairly dirty subway station, but you can't expect much more from the main hub of the TTC where thousands of people travel through each day!