Make or Buy?
Fundamentally, deciding on Excel resources is a classic make-or-buy decision. Reinventing the wheel usually isn’t the best use of your time. Using an existing resource may not be as much fun as spending hours mucking about with Excel (if you’re bent that way), but it can be a lot more productive.
On the other hand, if you wrote it, you know exactly what it does and how it does it (well, theoretically, anyway). You can tune your custom tool for the exact job at hand. If you can’t find an external resource that does exactly what you want, you may be better off "rolling your own," instead of trying to work around something that isn’t right in the first place.
However, even if you decide to write your own Excel resource, third-party resources can still be a big help. By examining the way in which other people have approached similar problems, you can pick up tips to help you do a better, faster job with your own custom resources.
If you’re really interested in doing your own Excel programming, consult some of the excellent references available from the Informit bookstore, such as Professional Excel Development: The Definitive Guide to Developing Applications Using Microsoft Excel and VBA.