I have tried plenty of GUI clients, but I am the fastest with git in the command-line hands down. git has quite a good user interface for command line use with its hunk-based staging, drop to $EDITOR to quickly edit a diff, etc.

Also, there is completion for git in shells such as bash and zsh.

Can you explain how ?

I use git on the command line too, it fits well with my workflow (branch features, "commit -a" when it seems relevant, rebase/squash when ready).

Maybe a more complicated workflow could justify an IDE plugin, but for me CLI is fine.