Depends on what your needs are. Do you want the coloring and grouping? Do you want Perl regular expressions? Do you want to only search source code? Do you want to be able to specify specific filetypes, like "ack foo --perl" or "ack foo --nohtml"?

git grep is neither superset nor subset of ack. Each tool has its place.

There is a large world beyond source code. Surely you've done some system administration or looked in logs?