As someone who isn't really great with public transportation systems or other modes of travel besides Lyft and Uber, this is truly as good as it gets.
When coming to and from Toronto, there is no easier way to get into downtown (Union Station ) than the Up Express.
It's simple: pay $12 CAD at the terminal, take it from Pearson for about three or four stops and you're there. To me, it's the easiest way to get close to where you need to be. If you need to take a cab from there, so be it. It won't cost you much more and the stress of figuring out how to get downtown is taken away with the Up Express.
It's about a 20 minute ride to and from with free wifi along the way. It's clean. Plenty of room and storage for your bags.
I can't recommend this train enough. If you're taking a cab or an Uber to / from the airport you're doing it all wrong.