I visited Toronto for a short stay and I used Union Pearson Express to travel from the airport to my hotel downtown. I went into downtown on a weekday, mid-day, and departed on an early Friday morning. For me, the 25-minute trip time was accurate both ways.
There was a short wait for the train, less than 15 minutes (helpfully there are signs letting you know the number of minutes to departure). The trains are clean, quiet, and spacious. Traveling with a large hand bag and a rolling carry-on suitcase was no problem as there is plenty of storage in luggage racks, overhead bins, and underneath the seats. The staff on board were friendly and efficient, and the free wi-fi on board is a nice bonus. I had purchased my ticket online beforehand, but you can also buy tickets at the station and on board.
I had a great experience and I hope the service continues. I wish every city had something like this to connect airports to their downtown centers.