In terms of pure utility, I'd give it 4 stars as Union Station is a very important passenger hub in southern Ontario. Customer service is very good, as a reviews have mentioned. Three major modes of transportation meet here: Toronto's TTC subway, the GO Transit regional commuter rail and the VIA inter-city rail. It even connects to both airports if you count shuttle bus services that stop nearby. And of course it's right next to venues for the major Toronto sports teams, some of the classiest hotels in town, the entertainment district and the financial district.
All that said, Union Station is in poor poor shape for a transport hub of its importance handling 41+ million passengers per year. The front facade and an expansive ticketing lobby can't hide the creaky, stained, grungy and utterly dated feeling that fills the entire space. The good news is the city is investing in long overdue renovations to expand and rejuvenate the facility including adding new retail spaces.
For inspiration, visit NYC's Grand Central Station, Chicago's Union Station or LA's underused but beautiful Union Station.