The #1 must have for comfortable and efficient browsing! There is no way I could use an internet browser without mouse gestures.
I generaly prefer a keyboard over mouse. However since most web pages are not usually keyboard friendly and I need to keep my hand on the mouse anyway, I mitght as well use the mouse the best way I can. Say goodbey to having to move cursor all over the place to switch a tab or navigate back in a page history. Scroll to the top or bottom of a page with a "wave of a wand". Close a tab? When I watch a person trying to hit the tiny x on a tab, it feels like ages no matter how skilled they are with the mouse. After a short while it starts to feel completely natural and you will want gestures to work everywhere!

There is still some room for improvements, compared to the now obsolete FireGestures plugin, that is. On the other hand, there are several nice features FG did not have. Anyway development seems to be active and I believe that most of the features I am still missing will be there.

For the FireGestures users, there is very little you will need to change if you used the default FG mapping.