Microsoft SQL Express offers so much when evaluating a persistence store for your own applications, especially if you are targeting Windows and utilising the .Net framework.
SQL Express offers pretty much most of the common features available in the full blown SQL Server which makes development and switching between SQL Server and SQL Express pretty seamless. However, there is one major factor which I would reccomend you consider… Deployment. You would expect the deployment of SQL Express to be as easy as installing say the .Net framework, which you will no doubt have to distribute.
In fact if you are targeting an internal business project or something with a fairly small consistant machine image (e.g. same operating system version) then you may not have too many problems, hey you can even set it as a pre-requisite to install along with the .Net framework as part of your visual studio deployment project.