I took the GO bus a lot in my 2nd and 3rd year. Nothing beats the convenience of driving to and from campus though.
There are express buses that leave from Square One that typically leave 5 to 10 minutes before the regular 47 that stops at Trafalgar. This is done to alleviate the seating issue that would otherwise occur.
Along with the express Square One bus, there is also an express bus from Trafalgar to Mac, but it only runs during peak hours.
All times are in the link I provided.