I used UP express to/from YYZ last week while in Toronto for the PanAm games.
I thought the price was a little steep - but not exorbitant, especially when considering the price of a cab for the same trip.
I arrived in Toronto at 7:30am on a Wednesday and needed to deplane; clear customs/immigration and be in downtown Toronto at 8:30. Thanks to no lines at customs and the UP express, I made it to my location with 15 minutes to spare. Worth its price there for sure.
I used it on the way back as well on a Saturday morning even though I wasn't in a hurry due to the convenience factor.
As I believe others mentioned, the seat was comfortable, there was room for my luggage, and I liked having an outlet at every seat + wifi.
If I returned to Toronto in the future, I'm not sure I'd use it unless I were in a hurry again to get downtown, but it certainly seemed like a good value.
I'd also echo whoever said that it was a little hard to find. I arrived at T1 on Air Canada, and followed the signs that said "train to city" assuming that was the UP Express - and it was, but I didn't understand why the signs also couldn't say "UP Express" somewhere. I had bought my ticket online ahead of time and knew I was looking for the UP Express, yet throughout the airport, all the signs had the generic "trains to city" label, which seemed very odd.
But all in all, a good value, especially if you're in a hurry.