Took the UP Express from Bloor station to Pearson when having an early morning flight that was scheduled before TTC subway service was starting to run on a holiday.
The UP Express is quick and clean, and runs from 5:30am til 1:30am every day.
My only real beef is that the signage wasn't great and I ended up going to the wrong track and then needing to run to the correct one to catch the train. Once you've navigated it, it should be easier but as a first time user I was a bit thrown off.
I'm a huge fan of the TTC Airport Rocket as it's actually surprisingly efficient for $3.25, but the direct route with just a quick stop at Weston on the Express is worth the extra couple bucks. Coming back from the airport when I'm on less of a time crunch, I'll stick with the Airport Rocket though.