Quickbooks pro is worth the money. I have been using it since 2000, and have up graded about twice since. Looking at 07 now to see if it is worth an upgrade.
You can customize QBs to suit your needs.
Printable invoices that are easy to customize for those that you bill. and if you bill them you can email them the PDF invoice. It will print out for them just as it looks for you.
A million different reports can be pulled.
When designing your invoice you can include some fields that don't some on the printed invoice, but help you track advertising, reminders, etc
If you buy it, before you start using it play with the sample company file for awhile to get your feet wet.
Then build your companies file, and then a 2nd to experiment with.
Community colleges after 2-3 days classes on the program also.