It's sort of like TreeStyleTabs, but nowhere near as powerful. With TST, you can configure the placement and look of the tab bar (or autohide it), group tabs into trees that you can collapse/expand/reload/close separate trees, and a number of other nit-picking options and actions that just make it feel right. Side tabs feels like an afterthought in comparison.

I mention this not to hate on Chrome or side tabs, but because I hope one day it will be as good. Once Chrome gets an actual equivalent version of TST and Pentadactyl, I'll happily switch.