Being a seasoned commuter now, I use this station quite often. I try my hardest to avoid rush hour times because I've seen lines for platform 46 go all the way past platform 44 and you know that you're not getting a seat on the bus when that happens.
If your bus happens to leave on platform 45 or higher, best walk down Yonge as opposed to Bay where it's busier.
There are numerous screens with bus times and platforms and GO is generally very good at being punctual unless there is a long line up, but there are large variances when it comes to when the bus actually pulls up to the terminal and people start boarding, I've seen some drivers leave it till 2 minutes before departure time to start letting people in.
There are 4 automated tickets machines (2 inside and 2 out) to go with the manned ticket counter and you can purchase your ticket using cash via the driver as well.
The station needs to drastically improve line management because you have 2 maybe even 3 lines at one platform when the going gets tough and a new rider is sure to get lost navigating these lines, the safest bet is asking more than one person in line what bus they are waiting for.