Certainly a beautiful idea, but given that "the 6.5km trial route from Stratford to Liverpool Street Station...they estimate would cost around £220 million" I'm not sure who would be happy to stump up the remaining funds needed for the entire 220 km proposed route...
This is the kind of idea that seems really cool until you start thinking through the practical aspects of it. I can't imagine your average citizen loving the idea of huffing and puffing their way up a long ramp to get 30 feet above street level. Once you're up there, except for high winds, it might be pretty fun and convenient. However, depending on how often there are entrances/exits (I would think not very many, given how much room a ramp would take up), you are then still facing the same old infrastructure on the streets you have to ride to your actual destination. And unfortunately, for all the money being spent on the "highway", it's one less dollar spent on street-level improvements, which will be needed in any case. I doubt this will ever be built in any case.
I want one along the Kennedy. To start.
I was thinking about designating key one way side streets for non-carbon emitting modes of transportation only. This would be much less expensive than a SkyCycle. I think it'd be really cool if certain streets were off limits to normal vehicular traffic, with the possible exception of allowances for local residential parking. These restrictions can be enforced with the installation of the soon to be ubiquitous, street surveillance cameras. In exchange for a convenience like that, I'd be willing to pay a bike tax if that were required to fund it.
A SkyCycle might be more appropriate for a city like London, with its loopy roads but Chicago streets are laid out in a grid pattern and I see no reason why all roads must be open to conventional traffic. Indeed, years ago, downtown State Street was only open to CTA buses and official vehicles.