I have only been using it for a few days, but it has already become an integral part of how I work each day. Springpad is great. I keep notebooks for the different projects I'm working on or topics that I'm studying. It's replacing Evernote and some of the functions of our household wiki. Great interface and amazingly consistent across platforms (iPad, iPhone, web,...). Did I mention that it's free?
They have a social element if you want to use it, a web clipper for grabbing stuff while you're browsing, email notes to yourself, share stuff with others, etc...
Some of the features I've just started playing with include a board layout for spatially manipulating items and product lookup for creating lists of things I want to keep an eye on.
Great tool. Highly recommended.