Bixi's an awesome concept.
I'm glad it's finally arrived in Toronto. My friend and I checked the map for docking stations today, and there seems to be one almost every other block from bloor to front, and between jarvis and university (?).
Here is how the payment system works.
You pay a flat rate to have access to the bike a set period of time ($5 for 24 hours).
You can take out a bike at any time during those 24 hours. The first 30 minutes are always free. Then you get charged increasing rates every half hour.
Basically, check in your bike every half hour to avoid paying anything except for the flat rate activation fee.
My one problem with bixis is that they are large, clunky, and heavy. They're not enjoyable to ride, and they are not fun for cruising. They're difficult to maneuver while pushing, and difficult to steer while riding.
Use them for transportation downtown, but if you're looking more for an enjoyable ride, bixi is not a good option.