They have an excellent FAQ.
I use both Furl and del.icio.us as they have unique features, I suppose if del.icio.us searched full-text, then I would go full-time
…I must try Spurl as it seems to have really usable features.
Although it would be logical to use My Web 2.0, I prefer to stick to the pioneers as I have more in common with the interests of the user base.
It seems that all these tools are maturing, and new tools are entering with all the required features, so now it’s becoming a factor of using the tool where the user base suits your interests.
…or a tool that is for a specific purpose or has a unique feature.