I am interested in FormsCentral and have read that you can customize the subject title for the Email submission receipt. What is not clear is whether you can use a response from a text box on the form as the subject title of the email.
Please advise as this is an important factor as to whether we purchase FormsCentral.
No, you cannot show the response in the email subject line.
If you mean customizing the email to client on submission, than the Subject of the email can be freely customized, manually. You can find it under the Options tab. The default settings for email subject is "Form Submission Receipt: «The Name of The Form»".