[batista]

I had used it before the HR, so it might have improved.

At the time It was like voodoo, and you had to triple-check your datastore actions, because they could fail for no reason at the backend.

[kroo]

App Engine's reliability massively improved with the HR datastore, and has gotten even better since the pricing change / SLA guarantee. It's actually remarkably good now, I recommend taking another look.