With the Java API you can write code that is standard (JDO/JPA for database stuff), that works across any Java environment. So it doesn't lock you in if you write your code in a smart way.
Except that writing code using JDO/JPA is a nightmare, at least on Datastore. So that's almost certainly not the smart way. I'd rather rewrite my app three times - go with Objectify (simple, easy, maps well to Datastore concepts) and just deal with the rewrite if it happens.
