The interface is great (while some design work might help). Seemingly trivial things like the search for an idea as I'm typing one is brilliant and keeps dups from showing up. I haven't done this in a collaborative manner just yet but am inviting a friend now.
This is a great service for free. I would consider paying for it if it really does help make the brainstorming and collaboration of an idea easier. The cost would have to be nominal though, since other methods still work even though they're might not be as effective.
Additionally, it would be nice to invite someone by email address in addition to a username. I have to email or IM my friend to sign up and then wait to get his username and then add him. Would be great if I could enter an email address and it would ask them to sign up and automatically add them to the idea.
Not sure I love the name though.