This is a great module for validating forms with Catalyst, especially when custom constraints are needed. The flexible interface allows you to set multiple named errors easily for more precise error messages. The FormValidator::Simple POD provides easy-to-understand documentation for integration with TT.
A nice enhancement would be the ability to define custom Catalyst constraint methods in the profile but I'm not sure if this is a good idea. I tried using Catalyst::Plugin::FormValidator which does allow custom constraint methods in the profile but had issues getting custom constraint methods to work in a Catalyst environment.