It's kind of hilarious how I was able to get from my condo in Toronto to my house in Taiwan without ever walking outdoors (except crossing the street once), kudos mainly to Union Pearson Express.
UP express is very efficient, clean and spacious. It beats taking a taxi on all fronts since there is one train every 15 minutes, is 1/3 of the price of a taxi (around $20) and avoids traffic jams. In a matter of 25 minutes, you are delivered to Pearson airport. It took me only forty minutes door-to-door to get to Pearson from my place.
Even if you have a lot of luggage it is not a huge problem as there are racks and overhead storage. The layout is like the GO train but with only 2 stops in between Union and the airport.
UP is amazing because it allows you to eliminate the stress of arriving late at the airport. It definitely made the whole travelling experience a lot more enjoyable for me.