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.
I feel like a broken record on this issue, but every few weeks I'll get fed up with something about FF and look to see if Chrome finally has a decent port of this and I'm always disappointed. One day, I hope...
I prefer them to scrolling when there are a lot of tabs open. Although Firefox's expose-style tab candy feature is nice enough that I don't get as many tabs stacking up as I might otherwise.
