I need Acrobat professional to create a fill-in-able form.
Yes, or Acrobat 5 of course.
If a user wants to fill in the form and save it, the user needs Acrobat
Yes, any full version of Acrobat (providing you have made the PDF backward compatible) or Acrobat Approval will do the trick.
If a user just wants to fill in the form and print it but not save it,
then that can be done with Acrobat Reader