So, let me see... on the negative side:
- have to use google-specific ORM code
- no joins or other "advanced" SQL features
- django admin interface does not work
- very limited set of libraries available
But hey, on the positive side:
- Dude it runs on google!
Seriously, are there really that many developers who can't figure out how to manage, deploy and scale their apps - who, by the way also have such huge scaling needs that they need to run on BigTable - that this fills any need at all? Because I don't see it.