I recently had to realize a project on Google AppEngine. At the beginning I was sceptic. But there a some really nice approaches on Appengine:
AppScale and TyphoonAE are both third-party implementations of the App Engine platform. TyphoonAE is targeted at small-to-medium scale, while AppScale is targeted at the large-scale end of things.
As far as backing up the blobstore goes, this is quite doable: just use the built in handler to serve blobs, and, in conjunction with remote_api, you can download your blobs just fine.