I would want to turn the question around. WHY on earth would you want to build your own?
If you need some extra fields, go with an existing package that can be modified.
Special report? Tap into the database and make it.
Believing that it isn't difficult? Try then. Spec it up, and see the list of features and hours grow. Then after the list is complete, try to find an existing package that can be modified before you implement your own.
In short, don't reinvent the wheel when another one just needs some tweaking to fit.
