The CN Tower is the tallest free-standing structure in the Western Hemisphere and had been the tallest in the world until 2010. With an observation deck at over 1,100 feet it provides you with unparalleled views of the city below.
The observation deck is very similar to many other observation decks around North America. Take an elevator to a high point and have 360 degree views of the city below (and around) you. There are some souvenirs for sale but that's pretty much all you get: views and not much else.
The observation deck does offer something pretty unique and that's the glass floor. No explanation needed here, it is what it says. Unfortunately, the glass floor isn't the entire deck or even a majority of the deck, its just a few panels off to the side. The view is cool and gives you some interesting perspective, but its not overly impressive in my opinion.
But you know what, if this is your first time to Toronto, the tower is a pretty incredible way to check out the city. You get a fantastic overview of the city and get to look out on the lake, watch the sunset, or just watch the city lights twinkle. Is this a must see? Probably not, but do yourself a favor and check it out at least once. Its worth the price of admission.