Any time you are considering developing a custom application it only makes sense to determine the best platforms for the application you need.  Some people think that because Excel is a spreadsheet application, this somehow lessens Excel's qualifications as a legitimate development application.   This is simply not the case.  Excel can be a very good development platform, depending on your needs.

Microsoft Excel is much more than just a spreadsheet. With the introduction of the Visual Basic Editor in Excel 97, followed by the significantly improved stability of Excel 2000, Excel became a respected development platform in its own right.  Excel applications are now found alongside those based on C++, Java and the .NET development platform, as a part of the core suite of mission-critical corporate applications.

Rob Bovey, et al, Professional Excel Development 2nd Edition

So how can you tell if Excel is a good fit for your platform?  The table below summarizes some of the issues you should consider:

platform selection factors
Issue Excel Access (Alternate Platform) Your Project's Requirements
User Interface needs to enforce complex data validation requirements Strong Possible to do, but difficult   Rate on a scale of 1 to 5
Need to peform complex and varied calculations on multiple fields Very Strong Possible to do, but difficult   (rate 1-5)
Leverage relationships between tables / data sets Terrible Very Strong   (rate 1-5)
Allow multiple users to edit records in the same table Possible, but it's not pretty Strong   (rate 1-5)
Copy and paste data from Excel into this new application Strong May have to paste one record or field at a time.   (rate 1-5)
Number of Records Up to 1 million rows Up to 2 GB file size.   How many rows/records will you need?

There may be other considerations that you identify that are key to your project's success.  Just add them to this table and rate Excel's ability with regards to the issue and the abilities of the other platforms you are considering.  Then just weigh your findings and pick your platform!