When to transition from Datastore to NDB?
To add to Dan's correct answer, remember ndb and the older db are just APIs so you can seamlessly begin switching to ndb without worrying about schema changes etc.. You're question asks about switching from datastore to NDB, but you're not switching from the datastore as NDB still uses the datastore. Make sense?
From what I have heard, it is better to move to NDB from Datastore. I would be doing that eventually since I hope my website will be performance intensive. The question is when. My project is in its early stages. 
Is it better to start in NDB itself? Does NDB take care of Memcache also. So I don't need to have an explict Memcache layer?