I use git-cola mostly when I want to commit various hunks separately; it makes that task real easy.
I was impressed by its equivalent functionality for the git --amend option the first time I saw it. If you click on the Amend Last Commit radio button, it actually displays the commit message of that previous commit. I was surprised to see it, especially since on clicking that option, I quickly ran to the command-line to copy that commit message, only to see the message waiting for me, ready to be edited away.
Another feature I really like is that if you attempt to commit something without staging it first, instead of just complaining, if offers to stage and commit all in one click. Nice.
These functionalities are real simple, but they really make for a pleasant user experience. Pity I haven't seen a tool as easy to use in the land of Mercurial. There, I use the command-line hg record which isn't as pleasant.