The CN Tower is a must-do for tourists, I think. I've lived in Toronto and nearby my whole life until recently and I only did the CN Tower once before, when I was a kid on a class trip. So I didn't appreciate it then like I did this time.
I don't really think the price is worth the experience. I mean, you're just seeing an Ariel view of a big city. It's not THAT amazing... but Toronto is a beautiful place (from up high) and Lake Ontario adds to the view massively.
I'd say if you have the money, do it once, but don't do it twice.