When customizing Dynaimics Ax a great deal of time is usually spent on testing that the customizations actually work. This is often a manual effort and can be quite time consuming.
On the Dynamics Ax application teams we are using various means of testing the standard application. Test automation is a key aspect and so is unit testing of the bits and pieces of new code we make.
In Dynamics Ax version 4 we have shipped a unit test framework as part of the MorphX environment. Partners can use this when they are developing their customizations and new functionality. If you are interested in how the unit test framework (SysTest) actually works you should check out the blog of one of my co-workers, David Pokluda (http://blogs.msdn.com/dpokluda/), who have just started describing this topic in details.