I started the vagrant-spec project to address this. A system like Vagrant is hard to fully unit test because there are a lot of side-effecty things that it does

I started the vagrant-spec project to address this. A system like Vagrant is hard to fully unit test because there are a lot of side-effecty things that it does. In many cases, the best way to test it is to run things in a real-life scenario. vagrant-spec includes a acceptance suite that we're going to be building up in order to test more of the features in a black box manner. It is slow to run but if we verify that things are passing before a release, we should be able to catch most daily use cases. Vagrant 1.4 is the first release to run through vagrant-spec and while there are some early bug reports in some fringe areas of Vagrant trickling in, Vagrant seems to be a drop-in upgrade for most users. This shows that it is working, to some extent. And it'll only get better. Of course, if you're nervous about it, it doesn't hurt to wait for a 1.4.1 or so on.