<p>I want to know if responsetimes are good, not just if it scales. Anyone with experiences?</p>

A scalable architecture like GAE is not the same as one based on optimizing speed. A different approach is needed.
The DataStore isn't designed to be super fast at the small scale, but rather handle large amounts of data, and be distributed.  We could say that database access is "very slow" at the small scale (compared with a standard relational database).
Take a look at:
google-appengine-second-look
and
how-i-learned-stop-worrying-and-love-using-lot-disk-space-scale
Some
experimental results
.