I've been submitting builds with On-Demand Resources for a few weeks now, some small and some multi gigabyte, with limited success. I've seen a few problems:
1) Getting a warning email from ITC that the app name is longer than 64 characters when it actually isn't. This usually puts the build into a permanent Processing state, which has lasted weeks. I haven't seen this since the GM releases of iOS 9 and Xcode last week, thankfully.
2) ODR was getting bundled into the app at download time in spite of being marked for later download. This, too seems to have disappeared since last week.
However, there still seems to be something in the ITC Processing phase that is not scaling well. A build with 1 GB of ODR might take 4 or 5 hours of Processing, but a build with 2 GB of ODR might take nearly 24 hours. However, it is much better now than it was a few weeks ago, so I suspect Apple is scrambling to fix these issues.