It is a small observation deck / area and totally worth it! It is another 33 stories up - that's another building! It's like taking the Westin Harbour Castle or the Commerce Court Tower and putting it on top of the main dome of the tower then looking down on the city from atop of that!
You'll probably never go up the tower again since most people only do things like that onces, so you may as well. Make it on a clear day if you can so the extra trip is worth it.
Considering what it costs to go to the observation deck, you're better of having lunch there and enjoying the view while eating, it's a way better deal all around. Plus, having lunch/dinner gets you free access to the observation deck.