Thompson House is the student lounge for McGill graduate students, It also includes a ballroom which can be rented out for events like weddings but you might have to be a member to take advantage of that. It comes complete with a bar and restaurant and is quite upscale for a student lounge. There are comfy armchairs and couches everywhere so it is quite common to see people coming here to work for a bit or just meeting friends for a quick drink.
Being a student lounge food and drink is very reasonably priced. The food is generally pretty good albeit the portions can be a little on the small side. I guess there has to be a tradeoff somewhere though. You can expect your typical bar food (pizza, sandwiches, etc) here.
The location is its downfall. It is north of Docteur Penfield on McTavish so it is quite the trek to get here. In colder weather this can be unpleasant. You also have to be a member or the guest of one to get in. The only way to become a member though is to be a grad student or post-doc at McGill.