For me, the easy capture via email or web, full text searching on JPG, PDF, DOC, ODT... and access by Win desktop, Web and iThing make it a winner.
I tried SpringPad but I wasn't interested in sharing a lot of information, nor its templates for storing different types of information, and found it was just too cumbersome to use for reference / GTD.
Another vote for Evernote - once you've got the web clipper installed it will save everything - a snapshot of the page, the URL, title, etc. It's all cloud synced and works on all the devices I need it to (well except computers running Linux - ahem). You can tag your posts which saves having to create the old folder structure that I remember creating under browser bookmark menus in days of old. Though of course it's powerful enough to do that too if you want. I now use Evernote as my complete reference store for more than just web clippings - but that's beyond the scope of what you're asking here.