People used to go to Buffalo for specific stores that were not available in Canada (like Victoria Secret, Abercrombie ect..). Now-a-days, there are very few companies that haven't expanded into Canada, and very few hot items that are only available in the states.
Now that Yorkdale makes the Galleria look like a dollar store, the only reason to go is to save a couple bucks on purchases (after factoring in a favourable exchange rate). Like you said, after you consider gas and the other costs, you would have a hard time saving more by shopping over the border than by staying local (assuming we are talking about the GTA here.