Bixi is a great idea. It's a fantastic idea. What it really needs is more stations, in more places.
Bixi isn't really useful if you're trying to go anywhere north of Bloor, west of Spadina, or east of Yonge. I think a lot of people don't really get the pricing scheme exactly - you pay $5.95 for unlimited 24 hour use, but only for the first 30 minutes per ride. Extra time on the bike is going to cost more.
The stations are kind of tucked away, and sometimes hard to find. A "Yonge" station will actually be tucked away 100 metres in some side street off Yonge.
The bikes are hugeee and clunky and not at all fun to ride - they have the high handlebars and low seats reminiscent of those beachy Schwinn Cruisers, but its not necessarily something I feel safe riding on the street in rush hour traffic. Turn radius sucks, and I feel like, to some degree, Bixi encourages riding without a helmet.
The phone support hotline is fantastic and always available.