I've attended a few events here now from a community group luncheon, a Stampede party, a weekend afternoon information session, to an evening celebration. Fort York is a great space for a mid size event (less than 100-125 people I'd say in the spaces I've been in).
The event rooms have character rustic charm and warmth. The catering is great whether it be a meal or reception type snacks/hors d'oeuvres . I find the staff to be friendlier and more welcoming than a lot of competing spaces (hotel venues etc) are. Judging by the variety of events I've attended, the clearly are able to cater to most budgets, whether it be corporate or non profit and the quality does not suffer.
There is a decent amount of parking available too and they are a quick cab ride from downtown - perfect if your party/event has out of town guests or if your event is right after work.
Events can be held during business hours or outside of the posted hours.