I started to use Radar Networks’ Twine at the invitation of CEO Nova Spivak after writing this earlier this year (also see this). I enjoyed it for a while, especially because a lot of technology folks were hooking up with each other, especially the semantic web community, on Twine. But I found it tedious to work through beta issues and to be bothered with recommendations or news about who was saying or bookmarking things about what. (I should have turned off the emails sooner!)
I was disappointed that Twine was taking an apparently folksonomic approach to tagging. It was as if Radar Networks was riding semantic web buzz without really embracing it openly or sharing the momentum that the invite-only community was investing in. That may not sound fair – I believe that there are semantics in the back room, but that’s how it felt and it’s still the way it looks. But probably the worst part is the process that you have to go through to add a bookmark – which is the whole point, of course! (I ultimately sacrificed popup blockers, but the process still seems laborious compared to other alternatives.)
I stumbled across Zigtag almost accidentally while working for a VC firm with a portfolio of semantic startups. What I like most about Zigtag is that they make it obvious that they are building an ontology of tags and encourage users to select semantic tags (i.e., concepts) rather than folksonomic “words”. They also provide tools for managing tags that allow you to move smoothly and incrementally from a folksonomic to a more semantic approach.