Short of begging a friend to drive you to the airport, this is the most reliable way to get to Pearson. You could also take a cab, but for the same price you can take a limo and ride in peace and quiet. You can't rely on the TTC, although you can totally get there by taking the 192 bus from Kipling station - you just never know what kind of delays you're going to encounter.
Did you know that the subway doesn't start running till 9:30am on Sundays? **
Because my flight was early I couldn't call on a vehicle-owning friend or make use of my TTC metropass, so I called Toronto Airport Limousine the night before to reserve a car for the next morning. There's a fare list depending which neighbourhood you're coming from and prices range from $45-65 (http://www.torontoairportlimo.com/rates.html).
My driver was 15 minutes early, and his phone call doubled as a wakeup call (I usually am just getting to bed at the hour that I had to get up). Sweet. I told him I'd need a few more minutes to get ready and he said that was fine. When I finally emerged from my apartment, he rushed to help me with my bags, asked me my airline and destination, and off we went.
22 minutes door to door from Leslieville. The car smelled nice, the driver didn't talk to me, and didn't once use a cell phone. He drove safely and swiftly and I wouldn't hesitate to use TAL again if I'm travelling at early hours on business.
**WTF.