<p>Does anyone have any experience migrating from one DBMS to another?  If you have done this, why did you do it? Features? Cost? Corporate Directive?</p>

<p>At times, I've worked with DBAs who insisted that we not use features specific to a DBMS (for example, CLR Stored Procedures in SQL Server.)  The DBAs point is, if we use these features, it will make it more difficult to switch to another DBMS if we have to.  But so far, I've never been asked to switch.</p>

In my opinion its silly not to take advantage of all the features of the db your using.  Changing DBMS regardless of how many features you use is going to be difficult.  There are minute differences between the systems (like some record Date and some record date and time) that will cause a huge headache to change.  There is no such thing of just switching to a new dbms.
From a business perspective there is a lot of work to be done.  Analysis on the new dbs to change to.  Figuring out the impact of changing dbs on the new system.  Having development change the existing systems, testing the changes etc.  The list goes on and on.  Making a switch like that on an enterprise system takes months if not years.  The last place I worked had to change dbs, and it took us a solid 11 months to do it and about 2 million dollars for consultants, hardware, software, and employee salaries.  It's a big deal.  If someone is saying not to use features, because that "may" happen someday and it'll be easier to do,  Most likely, that person is off their rocker.  The extra amount of time and money it will take to convert those features is minuscule compared to everything else (most likely).  IMO if it will save time and money now buy using those features, then that is the best course of action.
We did it because the systems we had running on the old dbms were too big.  There was too much data, and we needed something a lot more powerful.  Plus, it wasn't supported anymore.