At something like $18 per person when bundled with a Biodome ticket as of last week, I think they've reached a point where maybe the price has overstepped the value of the experience.
The elevator ride up is pretty cool, and the 270-degree view of Montreal once you get to the top is nice. Off in the distance, you can see the cluster of skyscrapers of the core downtown area alongside Mont Royal. That might be the best part of the view, although it's far enough away to be a bit underwhelming. A much better view of downtown is available by walking up to the chalet at Mont Royal.
Of course, that's not the fault of the observatory. The cityscape is what it is, and it's worth something to be able to see it from this vantage point. It's just not worth $18 per person, in my opinion. Still, if you're a visitor to the area you should do it just to have the experience.
Side note, it's evident especially on the elevator ride up and back, that this landmark has been falling into some state of disrepair (not in a dangerous way, but lots of old, peeling paint, etc). It's sad to see, because it's a beautiful piece of work.