Template feature for composing mail is really good. We can easily create template once and send similar mail to anyone in no time. 

However it would be very good if we can have canned response feature. So we save reply templates and when we reply to mail, we can select from drop down one of the templates and it automatically gets inserted in message body. We can make some necessary changes if need be and then send message. This saves lot of time specially when we have some predefined response for certain mails. 

Ref: http://gmailblog.blogspot.com/2008/10/new-in-labs-canned-responses.html