Ubuntus early chromium-browser package and PPA (by Fabien Tassin et al) worked out well for all, I think. The package couldnt be added to Debian so early for copyright reasons, and this early packaging provided an inspiration for the Debian packaging when it came.
Per-package archives with newer upstream versions of Debian packages are very useful. They dont exist within Debian because the infrastructure for that doesnt exist. Example: git.
I suspect (please dont flame me) that having python2.6 in Ubuntu early was a net positive for Debian and Ubuntu. Uploading to Debian experimental at the same time would have been better, since it would allow use of Debian's bug tracking system, give Debian developers a chance to help out with packaging details, and make it easier for many packages to be tested against the new version of Python.
Packages in "universe" are another story. Some Ubuntu contributors introducing packages there do not seem to have made any effort to contact the corresponding Debian maintainer. Yes, breakage sometimes results.
Some experiments do not seem to have been mentioned to the corresponding Debian package maintainers at all. Example: use of profile feedback based optimization for /bin/sh.
In general, DDs tend to be pretty open to experiments in Debian experimental, so it seems a shame not to take advantage of the usual benefits of an "upstream first" policy.