I live in downtown Toronto (and work downtown) and we use car sharing programs rather than owning a car - or will rent a car if we need one for a longer period of time. If you want to use a car for a full week straight - then it will probably be cheaper to rent the car from a car rental agency (e.g. budget, avis, etc.). If you just want to have a car from time to time (e.g. one or two hours at a time), a car share program will likely end up being cheaper for the first little while you are here.
In terms of car share being an alternative to buying a car - it depends on how often you use it. If you use a car daily (e.g. to get to work) - car sharing definitely isn't the right option. If you will only need a car time to time (e.g. every couple of weeks to run an errand) - car sharing might be a good alternative to the expense of car ownership. Ultimately you have to figure out what makes sense for you. If we had a car, we would likely use it one a week. So given the expense of buying, insurance, maintenance, etc. - it doesn't make sense for us. One of the other great things about car rental and car sharing - is that you get the car you need. For example, if we are just visiting friends outside of the city, we'll grab a small car. If we're picking big items up from a home store or something like that, we'll grab a van. It's great to have that flexibility.
You also want to be living in areas that have a many car share parking lots around you (i.e. several within short walking distance).
