The UP Express is now my favourite way to get to and from Pearson. For $9 each with Presto you can get to the airport quickly and in comfort, with free WiFi too!
Since the price drop, ridership has increased quite a bit but it's never crowded. The only difficulty is navigating through Union Station to get to the UP platform. Personally, I enter at the right side of Union Station opposite the Fairmont and walk along the Skywalk towards the platform.
The only way I think the service could be improved is if the trains didn't stop at empty stations - the journey would be quite a bit faster if it only stopped when there were passengers on each platform.
Remember that the prices are a lot higher without Presto ($12 each way) so if you don't have a Presto card, pick one up and load the card before you travel. There are also special fares for people going to and from the airport same-day within a set period of time (e.g. to pick someone up or if you have a layover and want to spend some time downtown).