What's nice about I Need to Read This is that you can use all of its services through bookmarklets instead of having to install anything in your browser. There's simply "I Need to Read This" and "Read an Article" bookmarklets, which you drag up to your browser's toolbar, and on any story you want to bookmark you just hit the former bookmarklet to save it. Then, to read what you have saved you click the latter "Read an Article" button, which takes you to the latest story. Clicking it again takes you to the second most recent, and so on.
Along with skipping to stories directly, the service maintains a complete list of stories you've saved, which is presented in reverse-chronological order. Like Read It Later, this is all saved in the cloud so you can access it from any computer, and because it's bookmarklet-based you could use it even from public computers without needing special permission to install anything.
There's not much more to it than that, which is what I like about it as a bookmarking tool. For power users, though, there is a Firefox Extension that gives you the bookmarking and quick reading buttons, right in your browser, although it's currently listed in the experimental section of Mozilla's add-ons.