Untill today I run only one site on a recent HEAD of drupal. And I am not at all happy with how things go there. This, I have to stress, is not because of certain features, or people, its because, I, (as a part of “we”) made some wrong decisions, that got us in a spiral of issues, everytime more issues, and so it got us stuck with a heavily delayed release.
The problem is mostly that I have to meet deadlines. That I have to make plannings and quotes based on estimated amount of hours. And I have found that this is near to impossible when you want to be involved in development for HEAD. The “things” you might encounter are too uncertain. The severety of the open bugs is too uncertain, to start building a tightly budgetted project on top of.