An observation deck mid-way through the building, just above the last office floor, would be nice, with a killer view of downtown nonetheless, and could probably house a restaurant, or something...well no... as far as i know, there are no observation decks in canada that come close to paying their way when you look at the dedicated space required for elevator access and exiting etc.
cn tower comes close and possibly husky tower but that's only because they're purpose built.
others - like the empire state building - that are successful on office buildings are in the heart of world renowned tourist centres. they're successful there and wouldn't be in smaller centres elsewhere (like edmonton - already proven with the closing of the telus' 33rd floor observation floor).
an observation deck on a residential building not only compounds the problems in general, it would considerably devalue all of the high end penthouse condos at the top whose patios etc. are now visible from a publicly accessible observation deck.