I just hope they had taken a little bit more care in ensuring backward compatibility - both my apps (which I have been running for 7+ months) lost code deployment ability (errors on deploy). The fact that it happened exactly at a time we discovered a major issue in our code base and were stuck without being able to push hot fixes was a major bummer.
Shot off an email to their support 2 hours back - haven't heard back yet.
Fortunately, we sniffed the issue to the new buildpack and found a work around by pointing our apps to use an older buildpack (https://devcenter.heroku.com/articles/buildpacks). While it caused us real heartburn for quite some time, kudos to them for building an infrastructure that is extremely configurable (which enabled the workaround).
Update: Heroku support got back with the same work around. They are debugging the issue with the new buildpack.