As for use_library() vs App Engine Patch: for me, the perf issue is primary. On the other hand, AEP offers a lot of functionality out-of-box that you don't get with raw use_library() django.
But after Google IO 2010 I have reconsidered. Google obviously has placed a firm commitment behind GAE.
Every once in awhile Django will barf but it always dies and restarts nearly instantly. If you know how to optimize your app for appengine, it's a very sustainable solution.
I've only used Python on appengine, so I can't really comment on your particular experience. It supports Java, and it's super-important to realize that you won't be working with a database, you'll be working with a dataSTORE (which is comprised of hash tables, sorta).
give it a shot. if you hate it, quit!