It's not super-cheap (but the price is much better with a Presto card), but it's so extremely convenient if your destination in Toronto is nearby Union Station.
The trains are very frequent, the trip is extremely pleasant and fast.
It's probably not the best value for money for everyone, but if you live in the area (that is if you have Union Station in walking distance) it's a service that's hard to beat when it comes to convenience.
We'll be using it every time we need to get to Pearson!