I have imported customer, inventory and serial number data in a variety of ways.
I started with QSCImport (the free utility from MS CustomerSource), bought the EMS MS SQL Data Import/Export bundle and have also used text functions in Excel to build a set of query statements.
I have had both success and failure with all three methods. Backup your database before importing!
Inventory - you need to update at least two tables (Item and Supplier) if you want the reorder info to be correct. QSCImport has some shortcomings but it does populate the Supplier table as well as the Item table in one process. It does have some limitations...
Customers - if you're just import names & addresses, any method will work just fine. If you're importing A/R balances, my experience has been that using any method other than QSCImport will not get the A/R data to all of the necessary tables. If you know which tables need what info, you could use another method but I'd recommend QSCImport if you have A/R data.
Overcoming QSCImport shortcomings - use Excel to build a list of Query statements. For example, the QSCImport utility won't import MSRP. After you've imported your data from Excel, create a text formula in a new column that makes a query like this: UPDATE Item SET MSPR = R1C1 WHERE ItemLookupCode = R1C1 (R1C1 would be a reference to another cell).
Errors - you will probably have some errors. If you can get the number of errors down to a reasonable number, it's easy enough to fix them one at a time. I have rarely had an import with zero errors.
Did I mention backup?
HTH, Tom