Hmm. I hadn't thought of balloons, to be honest. The biggest problem with zip-lines is getting people back up to the height of it for the next line - other than that, all you need are brakes to slow yourself towards the end of the line, else you're cliff (or in this case building) pudding.
It's been suggested before, apparently, from a cursory search. Elevators and stairs are too bulky to build everywhere, but with balloons, you could pump ballast up the pole to bring it back down to its lower station. This means it's two-way, a big plus. I don't know what the cost of running the pumps would be, but it couldn't be more individually than the maintenance that the buses require - relatively fewer moving parts, but perhaps a larger overall cost because of how many you'd need along one route. I don't really know how many buses our system operates, but it would also be much cheaper during down-times (no cost during the times the pumps don't need to run), and could potentially be automated, which means vastly extended AM service. It would have to be longer distances between each stop to be economical, but there's a certain distance where it becomes much less useful than a bus system.
High winds would be a big problem. So would less savory sorts trying to pop the balloons - the buses and bus stops are vandalized enough as it is, with drivers and cameras watching. I dunno. It's something to consider.