I open almost every link in new tabs. It's faster to close a tab and return to the prior tab than it is to go Back, and the back button often doesn't work with dynamic pages. Plus, when replying in various forums, it's much easier to flip between tabs to refer to stuff that was said upthread, especially when you have to go back a page or two.

Neither the back button nor bookmarks retain state sufficiently or have sufficient responsiveness to replace open tabs.