I don't really think the three-buses-wide balloon is a good idea for spying since everybody (including the radar) can see it coming, but I imagine this will be quite effective for search and rescue support, battlefield surveillance and all the other short-term missions that could put such disposable flying electronics better good use.
If I remembered it correctly, China Mobile has tested a drone-based base station which can provide cell service over a short period of time, and they were using the system during a earthquake rescue with some success. Maybe using balloon is an better idea.
PS. Funny thing. I was planing to build and release a water probe since I live very close to the Yangtze river. Thought I can probably measure the water speed with the probe by letting it send back it's GPS position, plus some pretty pictures via cell/SMS. I haven't started the project though, due to many hurdles. One of them is to figure out what trouble I can get myself into after the probe encountered something it shouldn't have (because I have no control over it after release). I don't really want to have a chat with CIA, or even worse, the water police (yup, and they got jet boats).