I have used Gmail (which is STILL in beta) now for well over a year, its quality I have 5 email accounts running on it, and the spam filtering is awesome.
Gmail has now added another new feature, you can set upp canned responses or auto responders as we normally know them as.
One ways is that it works like some of filters you may have already set up on your inbox. Step one choose the criteria based on keyword / people / subjects, step two choose what you want to do with them emails. See below image.
Google Mail Filters
You can now choose an auto response e-mail to send out. Great if you have many many emails coming in.
You can also choose your canned response right from the mail composition page (see below)
Canned mail composition
Images courtesy of the Gmail blog