ExtJS in general is a very convenient way of setting up communication between the interface and backend controllers of an ExtJS application, it takes all the RY (Repeat yourself) out of interface programming.
With the addition of CatalystX::ExtJS, the combination of ExtJS and Catalyst becomes extremely simple, very powerful and fun to use. I am able to create very complex applications in no time at all because both at the frontend (thanks to the excellent Sencha team) and the backend (thanks to this wonderful module) I no longer have to think about letting the two communicate, it just works. I migrated a very complex application to Catalyst in 1 day. Without CatalystX::ExtJS this would not have been possible.