If you are visiting Toronto, there are a few things the TTC is good at and a few that it is not the best at. Buses and streetcars are not the TTC's forte, but the Subway system lets you get around the city fairly well.
The TTC doesn't really keep to a schedule; busses and streetcars are often late, and by late I mean each and every single one can be late. For stops with frequent service (i.e. 6 minute service), it may be best to just wait outside. With that being said, the above ground transportation from the TTC leaves much to be desired.
If you plan on taking the TTC beyond its normal operating area (i.e. north of Steeles), you will get charged an extra fare.
With that being said, the best times to use the TTC include when you plan to go from one side of town to the other ($3 goes a long way) and if you are going to be using the Subway. The Subway generally gets you to where you need to go in a timely fashion (+/- 5 min generally).