Now that the price has finally been cut, the White Elephant Express is a decent value. They ran with a much higher fare at first, and the trains were empty.
The trains are clean, quick, and efficient, and with the revised fare structure much more worth it. Eventually they want to make better connections to the TTC at Bloor, but for now it's not a great link - if you're going uptown you may as well just take the TTC and save your money.
If you're visiting the city and going to be here a while, a Presto Card is a good investment, I think you can buy them at the airport - you should be able to...