If you are planning to take an intercity bus this is the main bus terminal in Montreal. From what I heard, this bus terminal was revampted a few years a ago. You will find there a coffee place, a small book store and ATM machines. It can easily be reached via the subway, city buses, by taxis and by car. There is even a bus connecting with the airport. The bus ride will take you between 40 and 60 minutes from the airport. By taxi it's more or less 20-25 minutes.
There are different ways to buy your bus tickets. Either the traditional wait in lign at the counter or by using the self service interactive terminal or on the bus station website. Worth to mentioned, their website is really bad! Try the website of the bus companies stopping there! (Limocar, Orleans etc..). They are much better! Just to give you an example, it not possible to find the address of the bus terminal on their site!! That's quite ridiculous.
Overall it's a good transit station. It's nothing fancy but it does the job! A small detail there is free wifi in the bus terminal and in most buses connecting to this station.