Its a neat enough idea. Would almost depend entirely on implementation. I am a bit skeptical such a verdant park as depicted in the render could be built without requiring fairly onerous engineering work to accommodate the necessary weight and irrigation requirements. If it just ends up like the ramparts at Nathan Phillips' Square, I'm not sure anybody would make the vertical hike to the top.
It might be a neat idea to make a kind of linear market, though. So, put retail along the central spine of the viaduct with pedestrian promenades along the edges. That should save on the infrastructure required to support a small forest. If it was designed well it could actually be a pretty cool attraction as well. I would like to sit on a patio above the Gardiner looking either to the lake or CBD. Lack of decent retail and commercial activity is the death knell of most waterfront projects.
EDIT: I would add that it is refreshing to see some actual creativity on this issue. As it is, I am sick of hearing dogmatic regurgitation of how the Gardiner is either the antichrist, destroyer of cities and eater of puppies or so impossible to rehabilitate that we ought not even consider it. We put a man on the moon for the love of god, it isn't impossible to do something with the Gardiner. I mean, I'm surprised this is lambasted as 'unrealistic' and far fetched, for a paltry price of 600m for 7km, while demolition of a well used route and replacing it with some million lane super boulevard (with a shrub median, lets not forget) for pretty much the same cost/km is actual municipal policy.