I'm a huge fan of BIXI. Toronto's Bixi system might not be as good as Montreal's (obviously) but for what it's worth, Toronto's is getting better. If they can expand into other areas and neighbours, it will be much more accessible and useful, especially for Torontonians.
I planned ahead and ordered a Bixi key (got it shipped to my cousin's place). So much more easier to just plug in the key and go instead of having to pull out your credit card and pay at the kiosk's every time. As I'm not from Toronto, I bought an occasional membership ($5) which allows you to just put in your key and then it automatically charges your credit card $5 (for 24 hour use!).
The Bixi hotline/customer service is great. If your from out of town, definitely try it out! So much fun and great way to get around the downtown area.