Personally, I recently had my fair share of problems with the DownloadError, but I knew it would come up (because I read the documentation - and I concur with the limit for scaling reasons). So I built myself a failover decorator relaying the failing requests to a VPS.
Otherwise, I love GAE!
1. For example the top commenter nl with 83 points: http://news.ycombinator.com/item?id=1928148