It's faster than ack (according to their GitHub page), and it honors the .gitignore/.hgignore by default, which is a nice feature!

With Ack, I had to set up a bunch of --type-add lines to ~/.ackrc so that Ack wouldn't ignore e.g. SCSS files. ag searches every file by default, except for VCS files (e.g. .git/ and .hg) and files that are ignored by your VCS.

Ag's default behavior is way better, and less confusing to new users. Plus, it's faster.

It seems to be a bit faster than ack (which is just called 'ack', Debian the package is called 'ack-grep' to avoid conflicts).